Loss of Fit for Sleeve Bearing.
Loss of Fit for Sleeve Bearing.
(OP)
Hiya Gents,
This is my first Thread since joining Eng.Tips.Read a good few posts to get my bearings(no pun intended).We have been having issues in our plant with clamping heads.The unit is simple enough,it consists of two small pneumatic cylinders bolted to an aluminium housing which is 250mm long,200mm wide and 175mm high.The cylinders are 180 degrees opposed,one on each side of the block and are actuated together to open and close.Attached to the end of the cylinder rods are machined plates with three shallow counter bores, one to accept the cylinder rod end shoulder, the other two to accept a guide bar and a rack which uses the same diameter and length of bar.The guide bar and rack bar run through phosphor bronze bushes(8 off) which are pressed into the aliuminium housing.These bushes wear pretty rapid(bi-monthly)so we are constantly replacing them but the press fit is lost after a few strip downs.I was taught it is not good engineering practice to press fit any type of bearing into aluminium for exactly this reason.I am new to the company and have suggested pressing permanent steel bushings in to house the bronze ones as replacing the alumnium blocks is really expensive.The other Mech. engineer on site whilst not knocking my idea is not sold.Has anyone had similar experience or could anybody provide an alternative solution.There are some world class guys here judging by the posts and replies so anything would be greatly appreciated.
This is my first Thread since joining Eng.Tips.Read a good few posts to get my bearings(no pun intended).We have been having issues in our plant with clamping heads.The unit is simple enough,it consists of two small pneumatic cylinders bolted to an aluminium housing which is 250mm long,200mm wide and 175mm high.The cylinders are 180 degrees opposed,one on each side of the block and are actuated together to open and close.Attached to the end of the cylinder rods are machined plates with three shallow counter bores, one to accept the cylinder rod end shoulder, the other two to accept a guide bar and a rack which uses the same diameter and length of bar.The guide bar and rack bar run through phosphor bronze bushes(8 off) which are pressed into the aliuminium housing.These bushes wear pretty rapid(bi-monthly)so we are constantly replacing them but the press fit is lost after a few strip downs.I was taught it is not good engineering practice to press fit any type of bearing into aluminium for exactly this reason.I am new to the company and have suggested pressing permanent steel bushings in to house the bronze ones as replacing the alumnium blocks is really expensive.The other Mech. engineer on site whilst not knocking my idea is not sold.Has anyone had similar experience or could anybody provide an alternative solution.There are some world class guys here judging by the posts and replies so anything would be greatly appreciated.





RE: Loss of Fit for Sleeve Bearing.
RE: Loss of Fit for Sleeve Bearing.
after repeated pressing in and out a bronze bushing from an alumnium housing, the press fit is lost.
first you didn't say how is this fit restored. I can't imagine how you are continuing.
If the bushing have a given OD, then the housing ID has increased, but you say the housing is too expensive to replace and is reused.
If so, I can only assume the bushings come oversized and you measure bore and machine to fit. Thus fit is not lost. but since the ID will be worn, ie not true cylinder, the housing will need to be bored also. This can be only so many times and then the housing must be replaced
from what I imagine, how about sleeving the housing with a harden steel liner. then instead of a press fit, go with size to size and a locking device. something as simple as set screws into a groove in the bushing's OD
Now, the original problem that needs to be adressed, why are you replacing the bushings so often. Given the design is aluminium housing with press fit bronze bushings, its designer did not consider maintenance would be needed. So was the designer in error or is there a operational problem.
one needs to look at the bushings and determine is failure is from wear or overload.
if wear, then look at surface finishes. the finsih needs to be applicable for the application, too polished won't hold lubriaction and too rough causes machining. You may need to add lubrication grooves to bushing IDs to act as a resovior and provide forced lubrication.
if from overloaded, it could possibly be an alignment issue, but more than likely it was under designed
RE: Loss of Fit for Sleeve Bearing.
I'd say move to a steel/stainless instead of aluminum for the press fit. If you can't replace the whole thing, can you modify them so you can add a stronger base material for the bearings to be pressed into. Ie the hardened steel liner mentioned by byrdj.
If you can get away with lubricating the bushings, that's a great way to go. You can cut a spiral groove in the ID, a couple small pilot holes through the wall and a groove around the od. The drill and tap some lubrication holes into the press-fit bores and pipe up to a spring operated lube resevoir.
You might actually be getting pre-mature failure due to the lack of a press-fit after the first few change-outs. This is a snow-ball effect. The first failure might've been a lot longer than 2 weeks, and it's just progressively gotten worse.
I don't know what kind of forces, cycle rates, etc you're dealing with, but something like an IGUS plastic bushing might be better than bronze for your application. You'd be suprised what they can handle.
James Spisich
Design Engineer, CSWP
RE: Loss of Fit for Sleeve Bearing.
Thanks,
Jamie.
RE: Loss of Fit for Sleeve Bearing.
ht
RE: Loss of Fit for Sleeve Bearing.
Jamie.
RE: Loss of Fit for Sleeve Bearing.
RE: Loss of Fit for Sleeve Bearing.
http://www.sdplastics.com/rulon.html
http://www.sdplastics.com/uhmw.html
RE: Loss of Fit for Sleeve Bearing.
Their T500 is their top of the line material for loads & wear. It's chemically inert and acceptable for use in food applications. You may look at your shaft composition and finish as well. Its only downside is it can be a little brittle so if it gets impacted it might fracture.
James Spisich
Design Engineer, CSWP
RE: Loss of Fit for Sleeve Bearing.
The chief engineer(my boss)is contacting a polymer bush supplier as we speak.The size of the bronze bushes were 25mm O.D.,40mm long with a 3mm wall so hopefully there will be no drama there.The shafting surface furnish is standard ground stock.I will research whether this needs modifying unless someone can answer on here.Yhe same applies for bearing retention.A top hat version would be nice so I could put 4mm countersunk screws through the flange wall to the housing block.T
RE: Loss of Fit for Sleeve Bearing.
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RE: Loss of Fit for Sleeve Bearing.