Basics | Design World Motion Handbook

How do gearmotors impact reflected mass inertia from the load?

January 10, 2019 By Miles Budimir Leave a Comment

In any drive system (that is, a motor driving a load), there is going to be inertia. Specifically, the motor will have inertia as will the load. For a direct drive system (a direct motor-to-load connection) the individual inertias can just be added together in a straightforward way. However, for any other type of setup involving some additional mechanical drive components such as couplings or gears, the relationship changes.

Pictorial representation of a drive system with gears. (Image via Omron Industrial Automation)

Specifically, when looking at how gears impact a drive system’s inertia, there are some basic ways to look at it. The basics; any gear will reduce the load inertia reflected to the motor by a factor of the square of the gear ratio. When looking at a gearmotor, this is basically just a motor with an embedded gearset in it, so the calculations and formulas for calculating inertia are the same.

In high performance motion control applications, it’s ideal for the reflected load inertia to equal the motor inertia. If inertia matching is the only concern, then the gear ratio can be calculated as:

N = √ (J_Load/ J _Motor)

where N is the gear ratio, J_Load is the inertia of the driven load, and J_Motor is the inertia of the motor.

Note: Another way to calculate gear ratio is by reference to the individual gears. For instance, if N_In is the number of gear teeth on the input gear and N_Out is the number of gear teeth on the output gear, the gear ratio is then:

N = N_Out / N_In

As for reflected inertia itself, this is best understood as the inertia of the load that is translated back to the motor through various drive components, in this case the gearing of the gearmotor.

So in a simple direct-drive system, the total inertia would be calculated like this:

J_Total = J_Load + J_Motor

However, for a geared drive system such as a gearmotor, the total inertia is calculated using the equation:

J_Total = ( J_Load / N² ) + J_Motor

This is because, as stated earlier, the reflected load inertia is equal to the load inertia divided by the square of the gear ratio.

Why is viscosity important for bearing lubrication?

January 10, 2019 By Danielle Collins Leave a Comment

Lubrication — or, more correctly, improper or insufficient lubrication — is one of the leading causes of equipment failures in industrial applications. Without it, sliding, rolling, or meshing surfaces experience significant friction, heat, and wear, leading to increased noise, loss of accuracy, and reduced equipment life.

One of the most important characteristics of a lubricant is its viscosity (or, in the case of grease lubricant, the viscosity of the base oil). But viscosity is analogous to friction in fluids. So why do you need friction (in lubrication) in order to reduce friction (in bearings)?

Tribology is the study and application of the principles of friction, lubrication, and wear between two surfaces in relative motion.

Viscosity = friction in fluids

Viscosity is a property of fluids and is caused by their internal resistance to shear. When a fluid is moving under laminar flow conditions, with no turbulence — as is the case with most bearing lubrication scenarios — microscopic layers of the fluid flow over one another, much like a stack of paper, with each sheet moving slightly faster than the one below it. Cohesive forces between these microscopically thin fluid layers must be overcome as the layers move past one another. The resistance caused by these cohesive forces is the primary determinant of the fluid’s viscosity.

In laminar flow, microscopic layers of fluid flow over one another. The layers’ internal resistance to this shear force gives rise to viscosity.
Image credit: Michael Fowler, virginia.edu

The role of viscosity in reducing bearing friction

Bearing surfaces, no matter how well they’re machined, finished, and cleaned, will always have asperities (peaks) and valleys. When two bearing surfaces are in contact (for example, ball and raceway or roller and raceway), the asperities of the surfaces interfere with each other. One of the primary roles of lubrication is to separate the surfaces and reduce or eliminate the interference of their asperities, which significantly decreases friction and wear.

But when the surfaces are stationary (or moving at very low speeds), the pressure between them essentially “squeezes” the lubrication from between the surfaces. There is a very thin lubricating film, but it is not sufficient to separate the asperities of the two surfaces, and significant contact still exists. This is referred to as boundary lubrication.

In boundary lubrication, friction, heat, and wear depend primarily on the interactions between the surfaces, although chemical reactions between the lubricant and the surfaces can also contribute to wear. The time spent in boundary lubrication conditions significantly affects bearing performance and life.

As bearing speed increases, lubrication is “pulled into” the contact zone.
Image credit: Wessex Petroleum Limited

As velocity increases, more lubrication is “pulled” into the space between the bearing surfaces, allowing a thicker lubrication film to develop. This causes the pressure on the lubrication film to increase, which, in turn, increases the lubricant’s viscosity (according to the lubricant’s pressure-viscosity coefficient). But there is a transition period where, as velocity increases, the surfaces are separated in some places, but in other places, interference between asperities still occurs. This is referred to as mixed lubrication.

Finally, when a sufficient velocity is reached, the lubricating layer becomes large enough to separate the asperities of the two surfaces, and the increased viscosity gives the lubricant sufficient film strength to support the load, by elastically deforming the bearing surface. This lubrication regime is referred to as elastohydrodynamic lubrication.

A Stribeck curve shows the development of the lubrication film and the resulting change in friction. As velocity increases, the bearing surfaces “pull in” more lubricant, creating a thicker lubricating layer under higher pressure, which results in higher viscosity. Finally, the layer is thick enough to separate the asperities of the surfaces, and the viscosity provides sufficient film strength for the lubricant to support the load.
Image credit: Anton Paar GmbH

Feature image credit: Exxon Mobil Corporation

Why is static friction greater than kinetic friction?

January 4, 2019 By Danielle Collins Leave a Comment

Solid surfaces are subjected to two types of friction: static friction and kinetic friction. Static friction acts when the surfaces are stationary — think of a box on the floor. Static friction is what keeps the box from moving without being pushed, and it must be overcome with a sufficient opposing force before the box will move. Kinetic friction (also referred to as dynamic friction) is the force that resists the relative movement of the surfaces once they’re in motion.

The static friction between two surfaces is always higher than the kinetic friction (at least, in practical, real-world applications). But why is this? To find out, let’s look at the causes behind each type of friction.

Static friction

There are several theories regarding the causes of static friction, and like most friction-related concepts, each one proves valid under some conditions, but fails under other circumstances. For real-world applications (especially those related to industrial machinery and motion control) the two most widely-accepted theories behind static friction have to do with the microscopic roughness of surfaces.

Static friction keeps the box from moving until the applied force (F) exceeds the static friction force (f_s).

Regardless of how “perfectly” a surface is machined, finished, and cleaned, it will inevitably have asperities — essentially “roughness,” consisting of peaks and valleys, much like a mountain range. (Technically the “peaks” are the asperities.) When two surfaces are in contact, it may appear that they have a large, well-defined area of contact, but in reality, contact occurs only at certain places — that is, where the asperities of both surfaces interfere. The sum of these small areas of contact between the asperities is referred to as the “real” or “effective” area of contact.

Because these individual areas of contact are very small, the pressure (pressure = force/area) between the surfaces at these points is very high. This extreme pressure allows adhesion to occur between the surfaces, via a process known as cold welding, which occurs at the molecular level. Before the surfaces can move relative to each other, the bonds that cause this adhesion must be broken.

Because the surfaces are microscopically rough, with asperities and valleys, the real contact area between them is much smaller than the apparent contact area.

In addition, the roughness of the surfaces means that at some locations, the asperities of one surface will settle into the valleys of the other surface — in other words, the surfaces will interlock. These interlocked areas must be broken or plastically deformed before the surfaces can move. In other words, abrasion must occur.

So, in most applications, static friction is caused by both adhesion and abrasion of the contacting surfaces.

Kinetic friction

Overcoming the static friction between two surfaces essentially removes both the molecular obstacles (cold welding between asperities) and, to some degree, the mechanical obstacles (interference between the asperities and valleys of the surfaces) to movement. Once movement is initiated, some abrasion continues to occur, but at a much reduced level than during static friction. And the relative velocity between the surfaces provides insufficient time for additional cold welding to occur (except in the case of extremely low velocity).

Once the static friction is overcome, the box will move. Now, the friction opposing movement is much lower and is referred to as kinetic friction (F_k).

With most of the adhesion and abrasion being overcome to induce movement, the resistance to motion between the surfaces is reduced, and the surfaces are now moving under the influence of kinetic friction, which is much lower than static friction.

Assumptions regarding friction

Friction is an incredibly complex force that manifests differently under various conditions, making it difficult to express in terms of physical laws and mathematic equations. However, there are three assumptions regarding friction that apply in most real-world situations:

The frictional force is proportional to the normal force.
The frictional force is independent of the apparent area of contact.
The frictional force is independent of the relative velocity.

You might notice that assumption #2 (friction is independent of area of contact) seems to contradict the idea presented earlier that because the points of contact between the asperities are very small, pressure between the surfaces is high and adhesion (via cold welding) occurs, which increases friction. But notice that assumption #2 specifies the “apparent” area of contact.

Using the box example from above: Assume you move the contents of the box to a different box with a larger footprint. The mass (and hence, the normal force) hasn’t changed, but the apparent area of contact is larger. Despite the larger apparent area of contact, the friction force remains the same, as predicted by assumption #2.

Why does rolling motion experience friction?

In theory, a ball makes point contact with the surface. But in reality, the ball (and/or the surface) deforms due to the load, and the contact area becomes elliptical.
Image credit: SKF

Note that the discussion above relates to non-lubricated, sliding surfaces. In theory, rolling surfaces, such as those found in most rotary and linear bearings (except plain bearings), should not encounter friction forces. But in real-world applications, three factors cause friction in rolling surfaces:

Micro-slip between the surfaces (the surfaces slide relative to one another)
Inelastic properties (i.e. deformation) of the materials
Roughness of the surfaces

What is current decay (aka recirculating current) in a stepper drive?

December 27, 2018 By Danielle Collins Leave a Comment

A common type of stepper drive is the chopper drive, so named because it rapidly switches the output voltage from the drive on and off — “chopping” it — to provide constant current to the motor. Chopper drives use one H-bridge per motor phase to drive voltage to the motor windings. A sense resistor at the bottom of the H-bridge monitors the current. When a set current level is reached, the H-bridge changes state to stop the current supply to the motor. After a set period of time (referred to as the “off time”), a new cycle begins, and the H-bridge once again drives current to the winding.

Although the H-bridge doesn’t supply current during the off time, current is maintained in the motor coils. While this stored current dissipates, or decays, it must have a path to flow through, or the field-effect transistors (FETs) in the H-bridge will be damaged. One option for a safe current path is to use freewheeling diodes in parallel with the FETs, but these add bulk and cost. Instead, the FETs themselves can be manipulated in one of two ways to provide a safe path for the decaying current.

An H-bridge is a circuit that contains four independently controlled switching elements (FETs) that direct the current flow to the load (in this case, the stepper motor).

In an H-bridge, diagonal switches are turned on to allow current to flow through. Alternating the sides of the “on” switches changes the motor’s direction of rotation.
Image credit: Texas Instruments

Turning on two FETs on the same side of the H-bridge causes very high current (referred to as “shoot through”) that could damage the FETs. Therefore, when the FETs on one side of the H-bridge need to be turned on sequentially, a method referred to as “break before make” is used.

This method disables the first FET for a set amount of time (typically a few hundred nanoseconds) before the second one is enabled, preventing shoot-through. During this brief “dead time,” when neither FET is enabled, internal body diodes on the FETs carry the current.

Current decay: Fast or slow

The two methods of current decay using the FETs are termed “fast decay” and “slow decay.” It’s important to note that, in this context, the terms “fast” and “slow” refer to the speed at which current decays, and are inversely related to how quickly the motor stops.

With fast decay, negative voltage is applied to the winding by switching on the opposite FETs and reversing the direction of current flow through the H-bridge. Fast decay enables the motor to respond quickly to changing step inputs, but it results in a slow stop of the motor. The downside of fast decay is that it causes high current ripple.

Although fast decay doesn’t provide truly recirculating current (current flows back to the power supply), it is sometimes referred to as a type of recirculating current.

With slow decay, two of the FETs (either both high-side or both low-side) in the H-bridge are turned on. This shorts the motor winding and allows the current to recirculate and slowly decay according to the motor’s L/R time constant. In a DC motor, the slow decay method causes the motor to stop rapidly because it shorts the motor’s back EMF.

Recirculating the current through the H-bridge provides slow decay (middle), while providing reverse current through the H-bridge to the power supply allows fast decay (right).
Image credit: Monolithic Power Systems

Microstepping drives find a compromise

Microstepping drives, which drive the motor windings with sinusoidal currents, typically use a method referred to as “mixed decay.” During the rising part of the current waveform, slow decay is sufficient to simulate the current sine wave as microsteps are issued, while providing high efficiency and low current ripple. But when the waveform is decreasing, slow decay doesn’t allow current to decrease fast enough to maintain the sinusoidal waveform, and fast decay would introduce unacceptable current ripple.

Fast decay produces significant current ripple, so microstepping applications use mixed decay.
Image credit: STMicroelectronics

Mixed decay solves this problem by beginning the downward part of the current waveform with fast decay enabled, and then switching to slow decay. This maintains the sinusoidal shape of the current while minimizing current ripple.

Feature image credit: STMicroelectronics

Electric slip rings: 5 things to know about brushes, voltage drops, and noise mitigation

December 27, 2018 By Lisa Eitel Leave a Comment

Here we outline five things to know about electrical slip rings and their brushes, voltage drops, and noise mitigation. Also be sure to check out the FAQs: What are slip rings and why do some motors use them? and What’s the difference between electromechanical and fiber-optic slip rings?

1. The three main brush technologies in electric slip-ring variations include monofilament, polyfilament, and composite.

• Composite brushes are made of carbon graphite sometimes with metals included to boost current capacity. Such brushes resemble those in brushed electric motors. Composite brushes excel where higher current and rpm are involved.

Composite-brush slip-ring image courtesy Deublin

• Other brush options include monofilament wire that partially wraps around the slip-ring drum at a channel — and is made of precious metal such as silver, gold, or palladium. These are common in low-current slip rings needing clean signal transmission and minimal contact resistance.

Precious-metal slip-ring brush image courtesy Deublin

• Polyfilament is also made of precious metal and partially wraps the slip-ring drum but includes multiple contacts per channel. Polyfilament brushes exhibit minimal contact resistance and noise, which makes them suitable for transmitting sensitive analog signals or data at high rates for real-time control.

2. Even the most basic brush-and-ring slip rings transmit power — but the selection of brush type is critical.

Parameters that dictate the most suitable choice include current, rpm, and temperature.

Note: Transmission of power also requires consideration of the voltage drop across the slip ring and actual current flow. Any drops affect available voltage at load … and power dissipation within the slip ring. The latter turns to heat and significantly impacts operating temperature.

3. Electric slip-ring signal and data-transmission rates climb higher all the time.

The way brushes for data create electrical-resistance variation during rotation degrades transmission quality. This variation depends on the brush contact mode and force, rpm, and temperature. One solution for advanced applications is to use multiple contacts for each channel — as in the form of polyfilament brushes — to reduce this resistance variation.

Bit-error-rate (BER) expressions quantify noise and reliability of data through a slip ring. Shown here: The Keysight Technologies DDR Bus Simulator generates Bit-error-rate (BER) contours to quantify reliability. Visit testandmeasurementtips.com or keysight.com for more on this.

No matter the brush form, rates and capacities are key slip-ring signal-transmission parameters. Many offerings with Ethernet connectivity transmit signals and data at speeds to 10 gigabit per second (Gbit or simply Gb) or higher. Common offerings today go to 1 Gb — a fairly standard benchmark — though even slip rings with this speed of transmission are still fairly specialized solutions.

Of course, suppliers underscore that most OEMs don’t even need 1 Gbit; slower 100 megabits per second (Mb) is sufficient. But as the number of sensors increases, and slip-ring-serviced designs become more complex, Gb-level speeds — in the form of Gigabit Ethernet (GbE or GigE) in many cases — will become more of the standard.

Note: Beyond signal transmission is the movement of data over a slip ring — a common manufacturer differentiation. This data movement is the sending of high-frequency information for higher-level controls and operational functions … often over Ethernet or one of the derived protocols such as PROFINET or EtherCAT.

Many such protocols enable control of various machine subsystems, support IIoT functionality, and allow use of all the data originating from the proliferating sensors on machines. Ultimately their spread will necessitate more (and faster) data transmission via slip rings.

4. Electric slip-ring data transmission requires shielding and mitigation of electromagnetic interference or EMI and noise.

Mitigation of EMI is sometimes expressed as electromagnetic compatibility or EMC. Transmitting data necessitates slip rings with higher bandwidth and better EMI mitigation than those transmitting power. EMI often travels via conduction and radiation. The former takes the form of detrimental high frequencies riding the line electrical supply or signal. In contrast, radiated EMI propagates electromagnetic noise through space to nearby equipment. Shielding slip-ring connections against such EMI is paramount.

Note: Radio-frequency or RF shielding is another feature common in slip-ring connections. Recall that even if EMI and RFI are used interchangeably, they’re different. More specifically, EMI is electrical noise of any frequency, while RFI is that from 20 kHz to about 300 GHz.

This is the electromagnetic-spectrum … with radio frequencies to right.

5. Electric slip-ring data transmission also requires grounding.

Assume a design engineer or OEM has a large electric motor that’s spinning some machine axis. That motor will have a drive such as a VFD generating a lot of electrical noise. Nearby slip rings that aren’t properly grounded will be affected by this noise; interference originating from that motor drive will couple to the slip-ring traces and introduce interference on communications or signal lines.

For designs where EMC is a design objective, a fully metal enclosure with a dedicated ground — along with only shielded cables — mitigate most EMI.

Note: Such preventative measures become more critical as the data speeds increase. That’s because at some point as the frequencies rise, data signals on the wires from the slip ring become emitters themselves.

Standard versus custom slip rings — and inductive, capacitive, wetted mercury variations

December 23, 2018 By Lisa Eitel Leave a Comment

Slip-ring variations abound — and the advanced components come in bore sizes from half an inch to many feet in diameter — for medical-imaging machines designed to service livestock, for example. But options go beyond mere scale. So how do OEMs and design engineers choose between standard and custom options … and what are other options beyond the most widespread variations of electromechanical and fiber-optic slip rings? Let’s explore.

Standard slip-ring options for OEMs

Some slip rings are sold as standard product offerings. These slip rings are easy to add to machine builds — with harnesses for simple connectivity as well as standard mounting interfaces. The latter can be in the form of a flange, threaded rotor, mounting on a shaft, or fitted into a shaft’s internal bore.

Standard hardware can serve a secondary purpose — ensuring slip-ring installation alignment to within tolerances for proper operation. Required shaft alignment is sometimes expressed to within some value of runout; slip-ring face perpendicularity to the axis is usually held to even tighter tolerances.

Video snippet courtesy Deublin. Click to watch the full version on YouTube.

Circuitry options include various standard contacts to satisfy amperage and voltage ranges — ac or dc.

For example, the manufacturer Deublin (known for supplying customs to the industry) is now getting into offering more off-the-shelf slip rings in standard product lines. This is an interesting departure from the general trend towards customization in industrial components.

Where might standard slip rings work? Well, an engineer might know he or she needs ten channels or ten individual circuits over a slip ring … and an off-the-shelf item will get this job done. Of course, there are other design considerations — and an engineer that chooses a standard slip ring may ultimately end up with a slip ring that’s a bit larger than needed … but in the end, the benefits of an off-the-shelf selection maybe worth the overbuild.

In other cases, engineers don’t always know upfront the number of dedicated circuits required — or Ethernet connections, for example.

EtherCAT image courtesy Beckhoff

Note: One factor related to Ethernet that often confounds engineers is that one basic cable has four 100-megabit Ethernet runs — containing only four individual wires. But the outside of that cable is metal braid for shielding. That metal braid counts as an additional circuit. So slip-ring manufacturer count five circuits or traces minimum for a standard Ethernet line … though many design engineers will think it’s only four.

Ultimately, issues like these underscore the importance of market education and communication between manufacturers and the design engineer on how to fit designs with slip rings.

It also illustrates the need for both off-the-shelf and custom slip-ring options … to serve design engineers with tight design cycles and those who need more involvement to develop very specific builds.

Custom slip rings

In many cases, trying to fit a standard slip ring to a design ranges from challenging to impossible. That’s in part because there are just so many things to consider in regard to slip-ring design … far more to consider than most engineers initially assume. Custom-designed slip rings are specific to design engineers’ specifications — pun intended.

Some customized designs that incorporate slip rings take different formats … for example, some slip rings ultimately come in unexpected housings … assemblies with square footprints, for example. Deublin supplied one such slip ring with an inner diameter of about 13 in. and the sides of the outer box were about 24 in. each … not a morphology most engineers associate with slip-ring designs. These are truly custom subsystems designed for a tight fit into the machine.

In fact, custom slip rings can include combinations of connection and trace types; encoders and other feedback devices; heaters, if needed; shielding for EMC and subcomponents rated for high temperatures, if those are concerns; and mixed-signal handling. Some manufacturers can deliver custom slip rings in a few weeks.

Slip-ring customization can include integration into rotary unions. Image courtesy Deublin

Note: Slip rings fully integrated into rotary unions can take customization a step further … to simplify installation and minimize maintenance. That’s in part because the connectors for electric and fluid-power hoses and cables can be designed to join and unplug together —with quick-connect hardware.

Don’t discount how complicated these subcomponents can be. Visit Design World’s connectortips.com site for more on what advanced connectors look like.

Example courtesy Deublin • Visit deublin.com/product-support/electrical-slip-rings for details.

Now let’s explore three less common slip-ring design iterations that provide still more options to design engineers looking to connect rotating and stationary machine-build segments.

Wireless alternative #1 — inductive slip rings

Not as widespread as electromechanical and fiber-optic slip rings but certainly commercialized, some slip rings use inductive coupling for the mode of transmission. Benefits include long-life friction-free operation with no arcing — helpful where settings include flammable materials.

How do they work? Well, inductive assemblies act as transformers but with relative movement between coils — called the primary and secondary in this context. Cores (usually of layered silicon steel) couple the coils. (By the way, the core material choice is to minimize the generation of heat from eddy currents.) The transmission of electrical power is constant, regardless of rpm or the position of the stationary and rotating coils. Some versions boost power transfer efficiency with self-harmonizing functions — useful in the face of fluctuating loads or settings.

One caveat: Most inductive designs are more suitable for the transmission of power or low-frequency signals (in the case of data transmission) than they are for high-frequency signal conveyance.

Where do slip rings based on inductive coupling work? They are found in the wind-power industry. Read more about this on Design World’s sister site windpowerengineering.com, which discusses iterations specific to turbines. Inductive coupling slip rings are also useful on packaging applications where axes run at high rpm.

Wireless alternative #2 — Slip rings that use capacitance

Note that there are also wireless slip rings that work by leveraging detection of capacitance. In contrast with inductive variations using magnetic fields from conductive segments, capacitive variations employ electric fields.

Mercury-wetted slip rings

Yet another type of slip ring is a mercury-wetted slip ring. Here, pools of liquid metal make the rotating connections. Mercury is a toxic heavy metal so not suitable everywhere — certainly not where food and beverage products are processed. But the material gives slip rings long life for both data and power transmission — along with low-noise operation.

Resistance through the contacts is less than one mΩ. Compare that to traditional brush-and-ring setups of 10 to 20 times that. Life is also longer with mercury-wetted slip rings — to a billion revolutions compared to tens of millions from brush-and-ring setups.

Where are slip rings used? Common markets and applications

December 22, 2018 By Lisa Eitel Leave a Comment

Slip rings excel in myriad designs that include a rotating base or platform — including industrial equipment such as index tables, winders, and automated welders. Wind turbines are another key application — as are medical applications for MRI and CT imaging.

MRI BioMatrix Technology image courtesy Siemens Healthineers

Slip rings also work at the pivot point of closed-circuit security cameras.

Courtesy PowerbyProxi (now of Apple)

Slip rings on amusement-park rides include those that transmit power, control signals, and data to and from the moving sections of carousels, Ferris wheels, and vertical-fall rides. Helicopters, cranes and elevators, generators, and rotating kilns are other examples. Let’s look at a few more in detail.

DoD photo: Navy Petty Officer 3rd Class Logan C. Kellums • Navy Petty Officers Melvin Bergado and David Hill-Destefano clear after attaching cargo to an MH-60R Seahawk on the USS Chung-Hoon in the Pacific Ocean.

Slip rings common in wind turbines

Wind-turbine construction on the rise globally, especially in Europe. One challenge is constant exposure to the elements … with only periodic maintenance to clean the turbine of debris and relubricate. That means slip rings must have long-term reliability — with durable contact construction to withstand even peak power output. Offshore turbines especially necessitate highly reliable slip rings due to the cost and time expenditures incurred should they need service.

Courtesy Nordex SE

Another challenge is satisfying the requirements of increasingly large and powerful turbines needing exceptional power transfer and transmission of signals for positioning as well as condition monitoring. This includes pitch control with power and commands (via signals over the slip ring) for blade information, hub and pitch motor adjustments, and power backup.

Courtesy Nordex SE

Deublin sells offshore wind-turbine unions that incorporate a slip ring and encoder … with on and offshore assemblies capable of delivering to 250 A — along with Ethernet or EtherCAT or PROFIBUS signals and rotary position tracking — even while rated to IP66 and resisting shock and vibration.

Deublin offshore wind-turbine rotary union

Integrated into a rotary union, these assemblies also accommodate hydraulics.

Slip rings in machinery for semiconductor processing

Within those machines, slip rings (as well as slip rings combined with rotary unions) often work on indexing platforms to process wafers mechanically and chemically …

… where manufacture can involve the application of materials and fluids.

Case in point: During the process of chemical mechanical planarization — a process that usually comes late in semiconductor processing, so must go quickly — integrated slip-ring rotary unions excel … especially with features to withstand the corrosive and abrasive setting.

Indexing image courtesy Nexen Group • Watch the manufacturer’s Design World webinar on demand at designworldonline.com/how-to-build-rotary-indexing-tables.

Slip rings in aerospace, defense, and marine

In the aerospace industry, spacecraft solar arrays use small slip rings that mount at their rotor to panel-moving actuator outputs. In a similar application in defense, slip rings work in armored-vehicle turrets to allow elimination of separate turret or gunner-station batteries. Here, a major environmental challenge is maintaining reliable power, data, and even video-stream connectivity even in the face of vibration.

DoD photo: Marine Corps Cpl. Alexander Sturdivant • Marines provide security with M1A1 Abrams tanks during breaching operations as part of a field exercise at Camp Lejeune, N.C.

Slip rings complete marine designs such as cruise liners, icebreakers, Coast Guard vessels, and offshore platforms. One design making heavy use of slip rings is radar towers.

Slip rings also support the functions of remotely operated vehicles or ROVs and other designs for underwater. These are not unlike submerged applications where the slip ring is exposed to oil — as in the machine-tool industry, for example.

Recently Deublin worked on a slip ring that needed an IP67 rating for submergibility to one meter. The sealing effectiveness was paramount. The manufacturer ran a sealed slip ring underwater and passed the test. To be clear, high seal ratings such as IP67 are usually custom specified.

NASA photo | The International Space Station is seen from Space Shuttle Discovery. Solar arrays power the ISS and rely on specialty slip rings for power and data transmission in the challenging vacuum environment.

Slip rings in the packaging industry

Let’s consider SRC and SRD electrical slip rings from Deublin. These are specific to machines to support the packaging of pharmaceutical, food and beverage, and other consumable goods. These are setup to be modular and configurable — with up to 40 electrical channels and top speeds to 250 rpm … which is key in packaging that relies on speedy throughput for profitability.

Uniloy injection-blow UIB 199 machine example courtesy Milacron

Some of the challenges are more specific: For example, machines involving plastics that need to be melted for molding or recycling might subject slip rings to ambient temperatures reaching 150° C.

SRD electrical slip ring from Deublin

Of course, slip rings that transmit energy will generate their own heat — a pertinent design consideration when already operating in hot settings.

One last slip-ring application: Synchronous electric motors

Slip rings find use in the startup of asynchronous electric motors — in the ac induction motors known as wound-rotor motors. Here, they don’t transfer power. Rather, they insert resistance into rotor windings.To be clear — a slip ring in a motor is not the same as a commutator. Yes, their designs and functions are similar. But a ring in a slip ring is continuous. In contrast, commutators are segmented. Read more on this topic at the post: What is a wound rotor motor?

In short, slip rings in wound-rotor motors have three circuits. The rings are on the motor shaft (isolated) and made of copper or an alloy.

Photos: WEG Electric Corp.

Each connect to one of three rotor-winding phases. Graphite brushes connect to a rheostat or other resistive device. When the slip ring turns with the rotor, the brushes ride those rings and impart resistance to the rotor windings. That brings rotor current more in-phase with stator current. This boosts torque output with relatively low current. After the motor start-up sequence ends (and the motor reaches full speed) the slip rings short out and lose contact (to avoid dings to efficiency and torque they’d otherwise impart) and let that motor run as a standard ac induction motor.

What’s the difference between electric and fiber-optic slip rings?

December 19, 2018 By Lisa Eitel Leave a Comment

Design engineers identify the need for a slip ring in the very first stages of the design process — as usually there are no viable alternative solutions.

Of course, while engineers know near the beginning of the design process whether they need a slip ring, the slip-ring design details are usually developed and finalized with the manufacturer.

The most common slip-ring assemblies use metal rings or traces on one subcomponent. Then these interface with graphite or wire brushes that ride the rings’ surfaces. So as that metal ring spins, transmissions conduct via the ring to the brush or vice versa.

Where the application needs multiple contacts — which is usually the case — more ring-brush pairs stack along the Z axis. In some cases, manufacturers make their designs more compact by stacking the brushes on two sides of the slip-ring assembly.

A webinar on this topic was presented live on Tuesday, December 18, 2018. Click below to watch it on demand.

Metal rings for signal transmission are sometimes plated with silver, rhodium, or gold; those for power transmission are sometimes brass or (for higher rpms) phosphor-bronze.

Slip rings that use fiber-optic technology

Fiber-optic slip rings are usually called fiber optic rotary joints or FORJs for short. Some manufacturers partner with suppliers to combine fiber-optic rotary joints with other technologies that are suitable for the transmission of high power … and recognize the technology’s place in complementing other slip-ring power and data transfer options.

Data signals transmit well over FORJs — with 100 Gbps a typical speed. Plus data transfer isn’t affected at all by EMI from high frequencies. FORJs also excel where data loss can arise from resistance — and on axes with exceptionally high rpm.

Remotely operated vehicle (ROV) example courtesy NOAA. Click the gif to watch the full video.

FORJs are not without challenges … in suboptimal or compromised installations exhibiting connector issues; signal degradation in the form of polarization, wavelength, return, and insertion-related losses; more challenging engineering for multiple channels (to avoid crosstalk and other issues); and the need for higher levels of protection should the slip ring operate in harsh settings. As always, the application will dictate which technology is most suitable.

Fiber-optic slip rings often allow multiplexing or muxing — the transmission of multiple signals or data streams at one trace and at once. That allows for the full use of the optical lines’ high speed. Systems combining signals often use time-division multiplexers, which gives each independent signal a time window at an output. Other systems often use what’s called wavelength-division multiplexing — which gives each independent signal on the line a set light wavelength for transmission. This in turn boosts line capacity, speed, and scalability.

Pancake slip ring designs — common morphology for FORJs

Sometimes FORJs take the form of pancake slip rings — also called face slip rings or just flat slip rings. In this arrangement, the rings of the assembly are like tracks on a record. Using this analogy, the brushes ride the platter as a record-player needle. When discussing slip-ring morphology, the design we usually assume is the common drum design … not pancake slip rings. We usually take this morphology for granted.

Pancake slip rings based on contacting electromagnetic operation work for relatively low-current applications. They can be less power dense than comparable drum-shaped options — exhibiting more capacitance and crosstalk — and must be protected from debris on its working surface. Brushes are usually optimized for low height, but that can mean the brushes have little travel or exert relative light pressure on the rings. The outermost rings exhibit faster wear than inner rings, so can be the limiting factor on life. But pancake slip rings obviously have shorter axial lengths than traditional slip rings with a comparable number of circuits. No wonder that slip ring designs that incorporate both fiber-optic data transmission and other noncontact electromagnetic power transmission have pancake morphology.

Side note on slip rings incorporated into rotary unions

It’s common for OEMs to specify slip rings with rotary unions where a machine build needs both … as they serve similar purposes albeit with different media. Recall that a rotary union joins a stationary inlet with a rotating outlet and contains fluid under pressure or vacuum to maintain that fluid connection.

Deublin supplies rotary unions that incorporate slip rings.

Sometimes called swivel joints, these rotary unions integrate slip-ring subsystems quite well. One off-highway equipment example is a rotary union with integrated slip ring that transmits hydraulic fluid between an excavator’s track drive and its cab — even though the cab of the vehicle may turn 360° without limit.

Rotary unions on off-highway equipment accommodate the transmission of signals and fluid power between cab and track base. Excavator image courtesy Caterpillar

Can mechatronics engineering help alleviate the shortage of skilled manufacturing workers?

December 18, 2018 By Danielle Collins Leave a Comment

If you went to college in the 1990’s or earlier and were interested in engineering, you generally had eight or ten fields of study to choose from:

— Mechanical Engineering

— Aerospace Engineering

— Electrical Engineering

— Computer Science (often included in the Engineering department)

— Civil Engineering

— Industrial Engineering

— Chemical Engineering

— Materials Science Engineering

— Nuclear Engineering

Those of us who landed in the automation industry in the past two decades typically came from either a mechanical engineering or an electrical engineering background. And while both mechanical and electrical engineering programs cover a broad range of topics within their respective fields, there has historically been little overlap between the two.

Case in point: Those of us from the mechanical side were great at calculating forces and inertias but had limited expertise in motor and control theory — much less the practical application of motors and controls to linear or rotational systems. And those of us from the electrical side were great at the hands-on integration of motors and controls, but typically lacked experience in the fundamentals of mechanical design, such as calculating beam deflection or tensile strength. If you found someone who had mastered both fields — mechanical and electrical engineering — they either had many years of experience under their belt, or they had, at some point in their career, been thrown into a “sink or swim” situation and learned out of necessity and self-preservation.

But the silos that were mechanical and electrical engineering programs are changing. There are now engineers coming out of both 2-year technical programs and 4-year degree programs who are educated and ready to hit the ground running with practical experience in mechanics, electrical hardware, electronics, controls, and software. This new, diverse type of graduate is the product of mechatronics engineering: a field of study that first became available in the late 1990s and has seen significant growth over the last decade.

The term “mechatronics” was first coined by an engineer at Yaskawa Electric Corporation, stemming from the combination of mechanics and electronics.

This diagram, developed by Kevin Craig, a leader in the field of mechatronics, shows the diversity of related disciplines that contribute to mechatronics.
Image credit: K. Craig

Mechatronics engineering programs can now be found at community colleges, technical schools, and universities. These programs range from two-year associate degrees or technical certificates with a focus on practical, hands-on training, to four-year degrees that cover everything from engineering theory to hands-on applications, to humanities and social-science courses that will give modern engineers the “soft” skills they’ll need alongside their technical expertise.

Regardless of the length of the program and the type of certificate or degree awarded, one thing the mechatronics engineering curricula all have in common is that they combine mechanical, electrical, and computer science disciplines with hands-on training in software, controls, sensors, and networking to produce students who can effectively work in advanced manufacturing facilities.

Mechatronics engineering is inherently tied to the manufacturing industry, although it also has application in other fields such as surgical robots, aerospace, and even agriculture. In fact, industry leader Siemens sees the need for mechatronics education as such as critical part of manufacturing’s future that it has developed the Siemens Mechatronic Systems Certification Program (SMSCP). This program, which can be integrated into existing programs at schools and universities, provides training and teaching resources to schools and instructors, along with certification for students to ensure they have the necessary competencies for specific job profiles in mechatronics disciplines.

The rise in popularity and availability of mechatronics engineering programs could be a godsend for the manufacturing industry. A recent study by Deloitte and The Manufacturing Institute found that the US manufacturing labor shortage could equal 2.4 million by 2028. Much of this shortage is attributed to three factors: the retirement of baby boomers, with fewer millennials to take their place; shifting skills requirements, which are increasingly dependent on technical training and technology degrees (although not necessarily four-year degrees); and a misrepresentation of manufacturing jobs, which will require a concerted effort from manufacturers to overcome.

The report from Deloitte debunks the theory that advanced technologies such as automation, AI, and IoT are eliminating jobs. Instead the report finds that more jobs have been created thanks to these technologies. From the report:

For more than two centuries, the manufacturing industry has adopted new technologies and provided new jobs for workers. Today, the industry is experiencing exciting and exponential change, as technologies such as artificial intelligence (AI), robotics, and Internet of Things (IoT) are rapidly changing the workplace. While some predicted that these new technologies would eliminate jobs, we have found the reverse—more jobs are actually being created.

The availability and attractiveness of mechatronics training and engineering degrees can provide a two-fold benefit to the manufacturing industry. First, it can entice students and younger workers to consider working in fields and industries they would not have otherwise considered. And more importantly, it can prepare them for the types of jobs that manufacturers are having the most difficulty filling — those that require a multidisciplinary approach to engineering, marrying traditional mechanical and electrical concepts with practical knowledge of software, motion control, sensors, and networking.

Gearbox service factor and service class explained

December 14, 2018 By Danielle Collins Leave a Comment

Sizing a gearbox (or gearmotor) for an industrial application typically begins with determining the appropriate service factor. In simple terms, the service factor is the ratio of the gearbox rated horsepower (or torque) to the application’s required horsepower (or torque). Service factors are defined by the American Gear Manufacturers Association (AGMA), based on the type of gearbox, the expected service duty, and the type of application.

Image credit: Cone Drive

While service factors may seem to be very specific, with thousands of combinations of gearbox types and applications each assigned its own numerical value, the criteria used to determine these values are based not on testing and empirical data, but rather on extensive review and analysis of gearbox manufacturers’ experience.

In general, the horsepower (or torque) rating of a gear tooth is based on the durability of the gear surface — its resistance to pitting — or on its bending fatigue. As the service factor of a gearbox is increased, the relationship between the gear teeth life (based on durability of the gear surface) and load is proportional to the increase in service factor, raised to the 8.78 power. In other words, if the service factor is increased by 30 percent (from 1.0 to 1.30, for example), the gear tooth life will increase 10 times (1.30^8.78 = 10.01).

To determine the gearbox service factor, start by consulting a set of tables or charts provided by the manufacturer, based on the type of gearing (worm, spiral bevel, helical, etc.). These tables list a wide range of applications (conveyors, cranes, winders, saws, blowers, etc.), each with (typically) three levels of service duty the gearbox is expected to see: zero to 3 hours per day; 3 to 10 hours per day; or greater than 10 hours per day. Each of these application-service duty combinations is assigned a recommended service factor.

Service factors by application type and gearbox service duty, per AGMA guidelines.
Image credit: Regal Beloit Corporation

Remember, the gearbox service factor is much like a safety factor to ensure the gearbox meets the application requirements, taking into account typical operating conditions known to exist for various types of applications. Once the AGMA-recommended service factor is determined, consider other, non-typical, working conditions that can cause additional stress and wear on the gear teeth, bearings, or lubrication. If any of these conditions exist, increase the service factor accordingly to ensure a sufficient safety margin and life of the gearbox.

Some conditions that may require an increase in the service factor are:

Elevated temperatures
Extreme shock loads or vibrations
Non-uniform loads (cutting versus conveying, for example)
Cyclic loads (frequent starts and stops)
High peak versus continuous loads

Once the appropriate gearbox service factor is determined, multiply the service factor by the horsepower (or torque) required for the application, and the result is the output horsepower (or torque) required by the gearbox.

How does service class differ from service factor?

In some cases, manufacturers cite gearbox “service classes” rather than service factors. Service classes are designated as I, II, or III, and are generally translated to numerical service factors of 1.0, 1.4, and 2.0, respectively, to be used in gearbox sizing calculations. It’s common that even if a manufacturer publishes service classes for general application types, they also publish the more specific service factors for specific applications as well.

Suggested service factors based on service class.
Image credit: STOBER Drives Inc.

Why don’t some catalogs list gearbox service factors?

Servo-rated gearboxes require detailed sizing to choose inertia, output torque, and speed for proper servo system operation.
Image credit: Wittenstein

Using service factor to guide the selection of a gearbox is appropriate for applications driven by traditional AC induction motors. But because gearbox output torque, speed, and inertia are much more critical for the proper operation of a servo system, sizing a so-called “servo-rated” gearbox requires a more detailed and exact method. For gearboxes that are used in servo systems, the primary emphasis in the sizing process is on required torque and inertia match.