Category Archives: MRO

Getting the Most out of Your Linear Bearings (Part 2)

In order to get the best performance and life out of your linear bearings, proper lubrication is key.

A lubricant formulated for rolling friction should be used with linear bearings. In applications where operating speeds are low and loads are light,  linear bearings can be used without lubrication at a greatly reduced life. However, it is never recommended to operate linear bearings without lubrication. To protect the highly polished bearing surfaces from corrosion and wear, a lubricant is required.

Where linear speeds are high, a light oil should be used and provision for re-lubrication should be made to avoid operating the bearings dry. For typical applications, a medium-to- heavy oil has good surface adhesion and affords greater bearing protection. Linear bearings 2 inches in diameter and above may use high pressure lithium grease such as Shell Alvania #2 for moderate speed applications. Lubricants containing additives such as molydisulfide or graphite should not be used.

lubrication

Getting the Best out of Your Linear Round Rail Bearings (Part 1)

In order to get the most life and best applications out of your bearings, it’s important to understand the size of the load, how the load will be applied and the length of the stroke. Applying too much weight to a load can significantly reduce the life and efficiency of your bearings. Also, incorrectly distributing the weight on the load can be harmful. In addition to some helpful design considerations, let’s take a look at the load considerations below.

Load ratings are the required design life, shaft hardness and bearing dynamic that affect the load and can be applied to a linear bearing. Two dynamic load ratings are given for each bearing size based on the rotational orientation of the bearing.

The normal load rating is used in applications where the orientation of the ball tracks relative to the load cannot be controlled. The normal load rating is based on a load imposed directly over a single ball track. The normal load rating shown in the specification tables is slightly greater than would be mathematically calculated based on one track loading, because it assumes that the load is shared to some degree by one or more of the adjacent ball tracks.

The maximum load rating assumes that the load is applied midway between two ball tracks as illustrated below. In this orientation the load is distributed over the maximum number of bearing balls.

The normal and maximum load ratings are based on a Rc 60 shaft hardness and a travel life of two million inches. For linear bearing system operating at less than full rated load, the Load-Life Curve may be used to determine the travel life expectancy.

An equivalent load value can be calculated when sizing linear bearings for applications at conditions other than maximum rating.

linear bearings

Ball Screw Solutions for Long Travel & High Speed

Design World’s always excellent Danielle Collins posted a great article about how Design Engineers can achieve both high speed and long travel with Ball Screws.

Didn’t know you could do that, did you?

Traditional solutions involve Belt Drives, Rack and Pinion Systems or Linear Motors. But each of those come with their own set of drawbacks. Ball Screws are an ideal solution for applications that are sensitive to thrust force or positioning accuracy

Yet Ball Screws are rarely looked at for high speed/long travel applications. Why?

Critical Speed

Any long cylindrical object will naturally sag in the middle. Add rotation, and you will get a whip effect, similar to a jump rope. The speed of rotation where that effect starts is the Critical Speed.

Obviously, one way to limit this effect would be to have a shorter unsupported span. But how do you limit the effect if your application calls for a long travel span?

The Solution – Ball Screw Supports

ball screw supportsSome customers have made their own ball screw supports, usually paired on either side of the ball nut in pairs of 2, 4 or 6, to reduce the unsupported distance, essentially quadrupling the critical speed for each pair used. Depending on the application, they can even allow you to select a smaller diameter ball screw, without compromising performance.

So, have you used this solution? Or do you have an application where it might be a suitable alternative?