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# linear cylindrical bushing

## linear cylindrical bushing

(OP)
Is there a criteria that determines the maximum clearance between a cylindrical surface and a bore that can be tolerated without binding? I have a vague recollection about of an equation with a friction coefficient, but I cant remember the context.

Rick Fischer
Principal Engineer
Argonne National Laboratory

### RE: linear cylindrical bushing

I think sufficient length to diameter ratio is mighty important in reducing the tendency to bind or jam.

### RE: linear cylindrical bushing

(OP)
Agreed. I am checking calculations on a device with a cylinder moving in a bore, and I need to quantify "sufficient" so we can say we looked at it and we're OK. I think I remember something from my distant past relating diameters, length, friction, etc but I cant find anything in any of my design books.

Rick Fischer
Principal Engineer
Argonne National Laboratory

### RE: linear cylindrical bushing

As a rule of thumb, a length to diameter ratio greater than 2 to 1 is considered the "Long Slider" condition.

### RE: linear cylindrical bushing

The 2:1 ratio is used with the length of bearing to the distance of the load from center of the bearing to prevent "stick slip". That ratio doesn't help with clearance. Many linear bearing suppliers list out both a tight clearance and a "compensated" or "free running" clearance. Which one is best depends on your application. For a carriage with 4 linear bearings, if you have precision shafting and precision alignment of the shafts to each other than all tight clearance bearings should be fine. But if there is any misalignment then having one or two of the bearings tight fit and the other two "compensated" (i.e. floating) would be better. One product that has some useful information is the Simplicity Bearings from PBC Linear.

### RE: linear cylindrical bushing

It isn't in a 'design' book.
It's a sophomore kinematics problem, usually presented as an analysis of a child's toy.
There may be an image of a monkey on a stick or something like that.

Mike Halloran
Pembroke Pines, FL, USA

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