linear sliding bearing design method

[gkan]

Hi everyone,

I have designed a few linear motion systems in the past with linear bronze/PTFE/cast iron bushings (just a plain bushing with a reamed hole in the middle) using H7/g6 fit. Most of them work just fine but a couple, especially when they are loaded off center, tend to lock or wear too fast. I used a very simple design, 2 shafts and 2 bearings (one on each shaft) or sometimes 4 bearings, depending on the size of the motion table. I was wondering if there is a reference or formulas on how to properly design such a system, because I have not been able to find any. For example:

What is the appropriate length of each bearing?
What is the maximum distance between shafts to avoid locking?
How much can you load the table in a distance off center to avoid locking?
What is the ideal x/y ratio for the spacing of 4 bearing?



Thanks in advance,

George

 

[MikeHalloran]

Four bearings is a bad idea.
Three is better.
The shafts have to be perfectly parallel, which is impossible, or something has to slide to compensate for changes in shaft center distance.
That can be done by allowing the carriage structure to bend elastically, or making the OD of the third bearing into a double-D shape so it can slide in a fork in the carriage.

Find a sophomore level engineering kinematics textbook. Virtually all of them will have a lesson and a worked example of a child's toy comprising a monkey that sticks or slides on a stick, or something that's similar. Another thing that's similar is all linear motion slides; you can predicut whether they'll bind or not with a little geometry and some reasonable assumptions about friction coefficients.



Mike Halloran
Pembroke Pines, FL, USA

 

[dvd]

This may be of interest:

http://www.pbclinear.com/Download/WhitePaper/Demystifying-the-2-1-Ratio.pdf