How big a guide bush & shaft

[weaston]

Does anyone out there use any general rule of thumb (besides "it looks about right") to determine the minimum Guide bush Ø and length to use in a machine design. Is there perhaps some basic calculation to get a ballpark min spec for the guide elements.

I recently designed a pneumatic press and found that the guide bushes (Sankyo graphite impregnated brass bush) I specified should probably have been bigger as the ram plate would not slide smoothly on the shafts and if there was an uneven load on the plate, it would deflect the shafts.

I would like to ensure that this doesn not happen again...

Thanks.

[easymike29]

Longer, perhaps, but not necessarily bigger.

Gene

[greg b]

There are a lot of variables in the question you asked,for instance is the bush a single only arrangement or are there 2 in the housing and how fast does it move.By the sounds of things with the deflection you are talking about it sounds like a single bush where the shaft at full extention may poke out a bit.As a general rule for the bush depending on the design I use 1.5 to 2 times the diameter of the shaft and at times 3 times depending on how far the thing sticks out and what pressures are applied to it,sometimes friction becomes a problem and the length and or diameter has to be changed to suit.Best regards Greg.