Does a low speed ball bearing need to be preloaded?

[Bockenstedt]

I am working on a low cost, low margin, high volume robotic's project. Tho make use of very small motors the system is heavily geared, and as a result the output speeds on the drive shaft and leadscrew assy are relatively low. They are 500 to 1000 rpm. The motion profiles are 1/3 , 1/3, 1/3, and so the acceleration ramp is not excessive, in the .18m/s^2 range. Historically the company I work for has not pre-loaded their bearings and been fine over millions of robotic cycles, but I am looking for a way to substantiate that the preload is not necessary versus saying it will probably work. If I can ditch the preload mechanisms it will greatly simplify my assy and reduce the cost. Bearing manufacturers tell you to preload, but I have not seen any standard way of determining if you really need it or not.

 

[Hippo41]

The need to preload is surely determined by the accuracy requirement of the job. Bearings will have clearance which (generally) you want to remove to improve robot accuracy.

Preload is also recommended by bearing companies when you have high accelerations to prevent skidding during start-up.

In your post you say that the current system performs OK, so "if it ain't broke, why fix it?"

If you want a cheap preload system have you considered (and is it practical to use) a simple preload spring or wave washer which is neither difficult to implement nor expensive?

Lester Milton
Telford, Shropshire, UK

 

[EnglishMuffin]

The usual reasons for preloading are accuracy and stiffness. I once analyzed a deep groove ball bearing to determine whether it could skid during initial acceleration. The acceleration required for skidding was astronomical and beyond the current capability of any practical electric motor, so I have always been rather skeptical of this particular concern in my field, but maybe it can happen with very small assemblies. The main cause of skidding in angular contact bearings is gyroscopic torque at very high speeds. Angular contact paired ball bearings often develop preload automatically at high speed because of the geometry of the race ways.

 

[diamondjim]

Bo,
As the English Muffin suggests, if you do not
need stiffness in your application, there is
nothing wrong in using a small amount of
clearance. The repeatability of the robot
should be the same if the same loads are
applied repeatedly. If you change the bearing
which has a different clearance, you would need
to recalibrate the robot for the clearance
difference.