Gearbox misalignment is widely recognized as the underlying cause for the majority of gearbox repairs and failures. It can degrade machine performance, reduce mean time between repair (MTBR), and decrease the operating life of the gearbox, motor, and driven equipment.

Types and causes of gearbox misalignment

Proper alignment between the shaft of a gearbox and the shaft of a motor or driven piece of equipment (such as a conveyor or actuator) means that the shafts are collinear—that is, they are positioned in a straight line. Shaft misalignment can be parallel (also referred to as “offset” misalignment) or angular. Parallel misalignment is the distance between the shafts’ axes of rotation, measured at the plane of power transmission (typically the center of the coupling), and is expressed in mils (1 mil = 0.001 inch) or mm. Angular misalignment is the difference in the slope of one shaft relative to the slope of the other shaft, expressed in mil/inch or mm/100 mm.

Shaft misalignment can be parallel (left) or angular (right).
Image credit: Rexnord

Gearbox misalignment is often a result of installation and setup activities. But even if the alignment is within acceptable tolerances during installation, operating and environmental conditions can eventually cause the shafts to move out of alignment. For example, loads caused during operation and the transmission of power through the gearbox can cause shafts or other gearbox components to deflect. Thermal effects can also cause expansion or contraction of the shafts or gearbox components (especially the gearbox housing), causing the shafts to become misaligned. In extreme cases, misalignment can be so severe that bending stressing on shafts cause them to fracture and break.

The faster a shaft rotates, the smaller the acceptable misalignment value will be.

Indications of gearbox misalignment

Signs of gearbox misalignment include damage to seals or couplings, decreased bearing life, gearbox vibration, and damage to the gears themselves. If there’s significant angular misalignment, the first sign will often be seal leakage (especially for pumps and equipment with shaft seals) due to the seal being deflected by the misaligned shaft. Another result will often be excessive preload on the bearings supporting the shaft, although this may not become evident until long after the misalignment occurs. These excessive bearing loads will ultimately manifest as pitting of the races or balls and possible cage failure. Damaged bearings will eventually cause excessive vibration. Similarly, the gears themselves may also become pitted due to increased loads between the gear teeth.

Gearbox shafts are often connected to other equipment via flexible couplings, which can compensate for small amounts of misalignment, but good design practice doesn’t rely on the coupling to compensate for misalignment. Proper gearbox alignment can be achieved in some cases with dial indicators, but laser alignment tools are much more precise and less error-prone. There are no industry standards for shaft alignment tolerances, but manufacturers often specify maximum misalignment values based on testing and real-world experience.

Image credit: Apex Dynamics, USA

Selecting a gearbox goes beyond just determining the required speed and torque output. Duty cycle, radial and axial loads, inertia, and environment all play a factor. Not to mention the gearbox type (inline, right angle), precision level, and service factor. Properly sizing and selecting a gearbox requires consideration of all these criteria, but with numerous manufacturers and thousands of gearbox types and sizes available, it can be difficult to determine just where to start.

This is where manufacturers’ online sizing and selection tools come in. As shown in the examples below, these tools can provide significant time savings for designers and engineers by narrowing the field of choices to those that meet the most critical application criteria.

Gam Sizing Tool

Gam’s online sizing tool is user-friendly, with a drop-down menu of pre-programmed motor manufacturers and models. Once a motor and model are selected, the rated speed, peak speed, inertia, and motor dimensions are pre-filled. The user manually enters the application’s continuous torque and peak torque values.

Based on this data, a list of suitable gearboxes is provided, and the results can be narrowed further based on criteria such as gearbox type (inline or right angle), output type, maximum backlash, allowable radial load, and allowable axial load.

After the gearbox selection is made, the full model and description are presented, along with download options for CAD files and 3D models, a request for quote option, and a list of related products such as couplings and similar gearboxes.

Gam also provides a tutorial on how to use the sizing tool.

Wittenstein (alpha) Sizing Assistant

The first step in using Wittenstein’s Sizing Assistant is to decide whether you want to perform the gearbox selection based on the application data or on the motor being used. When “application data” is selected as the input criteria, the user enters the application’s maximum torque and chooses from pre-defined motion cycles (profiles) and operating time. The gear ratio is also entered manually, and the program provides a graphical list of gearbox that meet these criteria.

Filters such as output type, gearbox type, maximum backlash, and radial and axial loads help to narrow the possible options. A nice feature of the options list is that each gearbox also shows a “utilization” percentage, which essentially indicates how much of the torque capability is required by the application (considering the service factor).

Using “motor” as the input criteria only requires selecting the motor manufacturer and model from drop-down lists and manually entering the desired gear ratio. Again, a list of suitable gearboxes is shown graphically, and filters can be used to narrow the choices. Wittenstein’s software also lets the user compare gearboxes in a side-by-side list.

Once a gearbox is selected, the software provides download options for CAD files and data sheets, as well as the option to request a quote.

MicronMOTIONEERING

Micron’s gearbox selection tool gives three options for finding the right gear reducer: by application data, by Micron model number, or by competitor’s model number.

For searching by application requirements, the user chooses the gearbox type (in-line or right angle), operation (continuous duty or cyclical), maximum backlash, and gear ratio.

Then the number of cycles per hour, torque, and speed can either be entered manually, or they can be calculated by the software based on a user-defined motion profile. After the move profile is defined, radial and axial loads can be entered (if applicable), and the motor manufacturer and model are selected via drop-down menus.

The software provides a list of gearboxes that meet the application requirements, along with list price and lead time. The user can then choose a gearbox and download a CAD model or request a quotation.

Selecting by Micron part number is extremely straightforward, requiring the user to choose the Micron model, gearbox type, frame size, and ratio. Then the user either specifies the motor information by either choosing from a list of Micron motor mount kits, choosing the motor manufacturer and model from a dropdown list, or entering the motor dimensions manually. The gearbox information is presented in the same format as above.

The competitor replacement functionality is useful for interchanging a competitor’s gearbox with the most similar Micron version, although it requires sufficient knowledge of the competitor’s part number in order to provide a cross-reference.

Washdown processes are common in the food and beverage and pharmaceutical industries, where cleanliness of the manufacturing and handling equipment plays an important role in the quality and safety of the product. In fact, the US Food and Drug Administration (FDA) and the European Hygienic Engineering and Design Group (EHEDG), along with other government and industry-specific agencies, provide regulations and standards for the safe and hygienic processing, handling, and packaging of food, beverage, and pharmaceutical products. However, these standards often address overall machine or process design and leave the choice of individual components, such as bearings, motors, and gearboxes, up to the equipment designer and manufacturer.

Washdown applications introduce two significant hazards to the performance and life of a gearbox. The first being that the washdown solution could make its way inside the gearbox and damage the internal gears and bearings, and the second being the possibility of corrosion due to exposure to water and chemicals — specifically alkalines, chlorine, and acids.

Fortunately, gearbox manufacturers have found ways to address these hazards, and it’s now common to see “washdown” and “hygienic” designs among gearbox manufacturers’ standard offerings. But what features make a gearbox suitable for washdown or hygienic applications?

The pressures and chemicals used in washdown processes create challenging conditions for gearboxes.
Image credit: Kollmorgen

Gearbox materials for washdown

First and foremost, the materials used for the housing and other external components, such as the output shaft and hardware, are critical to making a gearbox suitable for washdown duty. While some equipment manufacturers choose to use gearboxes with epoxy painted housings for washdown applications, these can chip and need to be repainted periodically. In most cases, stainless steel is a better alternative than painted options.

Both 304 and 316 series stainless are suitable for washdown and hygienic applications, although 316 has better resistance against pitting, especially when exposed to the caustic chemicals found in many washdown solutions. However, the use of stainless steel presents a higher up-front cost than painted designs, and 316 series stainless even more so than 304. To mitigate the cost, some manufacturers use 316 stainless for the housing and 304 stainless for other components with less exposure (such as shafts and some hardware).

NORD DRIVESYSTEMS two-stage helical bevel gearboxes come in 3.58 to 61.88 ratios and transmit torque from 50 to several hundred Nm even while maintaining efficiency. Unicase gear housings and large bearings let the gearbox withstand high radial and axial loading on the output shaft. Some variations include NORD’s nsd tupH corrosion-resistant treatment of aluminum housings to withstand washdown.

Geometry to maximize gearbox cleanability

Another important design aspect of washdown or hygienic gearboxes is their shape. The purpose of washdown is to clean the equipment and prevent bacteria from growing, but if the gearbox has crevices, seams, or grooves where particles can be trapped, thorough cleaning will be difficult or even ineffective. This is why many gearboxes designed for these applications have a round housing with smooth surfaces and no external seams. And if corners are necessary in the design, they’re executed with a radius rather than sharp angles.

Gearboxes for washdown or hygienic applications are typically made of stainless steel and have a smooth, round housing.
Image credit: Wittenstein

Gearbox lubrication and seals

In the ideal world, lubrication would never make its way out of the gearbox and into the process, but in the real world, it can (and does) happen. To prepare for this possibility, applications that pose the risk of incidental contact with the product should be lubricated with NSF H1 food-grade grease or oil.

The primary task of gearbox seals is to keep lubrication inside the gearbox (see above) and keep liquids and contamination out. Washdown conditions make this especially difficult, since the seals must withstand the high pressure of the washing operation and be impervious to harsh chemicals. This is why it’s common to find gaskets or o-rings on the input side of the gearbox and two-part seals on the output shaft. Most gearboxes for washdown and hygienic applications use seals made of Viton™, due to its broad chemical compatibility, ability to withstand high temperatures, and excellent mechanical properties.

The most common system used to classify a product’s washdown compatibility is the IP rating, which is published by the International Electrotechnical Commission (IEC). (IP stands for International Protection, but is sometimes referred to as Ingress Protection). IP ratings that convey suitability for washdown are:

• IP65: protection against water jets
• IP66: protection against powerful water jets
• IP67: protection against immersion, up to 1 meter
• IP69k: protection against powerful, high-temperature water jets

The National Electrical Manufacturers Association (NEMA) has also developed and published protection ratings, but NEMA ratings are used almost exclusively in the US, while IP ratings are used internationally. It’s important to note that there is not a direct correlation between NEMA and IP ratings.

Feature image credit: Kollmorgen

In this feature we focus on closed gearbox designs for washdown, but open gearing is particularly easy to sanitize. Here, KLEENLine retractable conveyors use precision-machined PowerCore gears from Intech Corp. to replace stainless-steel pinions in their rack and pinion drives. That eliminates the need for lubrication after washdown.