When specifying a ball screw, understanding the application criteria that the component needs to satisfy is essential. There are several factors to consider and each one plays a role in the decision-making process.
One approach is to think of the acronym LOSTPED to help evaluate the application.
LOSTPED stands for:
Load – including the mass being moved and the forces involved in the machine’s processes
Orientation – for example, horizontal or vertical, which affects the need for load bearing support
Speed – velocity and acceleration
Travel – the stroke length
Precision – includes positional accuracy and requirements for repeatability
Environment – special considerations for rugged or unique environments
Duty Cycle – cycle rate
Precision and Preloading
Ball recirculation inside the ball nut can affect precision and repeatability.
As a result, ball nuts are available with a range of pre-load options to reduce or remove the axial play as they rotate around the screw. This allows for better repeatability, for example, because no motion is lost from the clearance in the balls as they reengage.
There are several techniques for pre-loading.
Some common methods include oversizing the balls inside the nut housing; using the so-called “double-nut” or “tension nut” method; or by using a manufactured offset in the raceway spiral to change the angle of ball engagement (the “lead shift” method) and deliberately force the balls into a pre-load condition.
Each method has its advantages and disadvantages, but all serve to minimize or eliminate backlash between the nut and screw.
Speed Considerations for Ballscrews
There are two different speed factors to consider when it comes to sizing ball screws; characteristic speed and critical speed.
The characteristic speed depends on the limitation of the bearing system, and what speed it can withstand. This is an industry standard for a variety of bearings, indicating speed limits that should not be exceeded in operation.
Controlling criteria are primarily temperature levels, vibration or other operating limits of the bearing, and inertia of the moving components within a bearing. By optimizing a particular design, manufacturers can improve the speed factor.
Critical speed is essentially the rotational speed at which the ball screw will start to vibrate and introduce excessive oscillation (known as screw whip). This is heavily dependent on the length of the screw along with the mounting support for the end bearings.
Critical speed is a result of physics alone. However, there are a few things you can do to make sure that the screw speed remains outside the critical speed range. These include increasing the screw diameter, choosing appropriate end bearings, and using screw supports.
Because ball screws are a bearing system, they’ll need some type of lubrication to avoid metal-to-metal contact of the balls in the raceway. While the lubrication choice can be either oil or grease, it’s advisable to avoid solid additives (such as graphite) as they will clog the recirculation system.
An NLGI no. 2 type grease is recommended but it should also depend on the application, whether food-grade or another special type of lubrication is required.
The lube amount will be fixed, but the frequency of lubrication will vary depending on factors such as the move cycle characteristics, or contamination in the environment.
Read the related article: What are Ballscrews? Summary for Motion Engineers
Also check out: Simplifying Machine Design with Plug-and-play Motion Subsystems