Loss of Fit for Sleeve Bearing.

[ukjay]

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.

 

[sreid]

Are the bushings oil filled porous [sintered] Bronze?

 

[byrdj]

Is this what the question simpilfies to.

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

 

[DesignerGuy16]

If you can deal with that problem and can't replace the plate you're doing it wrong. You have a two-fold problem with a weak substrate you're repeatedly pressing in and out of, and you're failing bushings too frequently.

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

 

[ukjay]

Thanks ever so much for all your input.Really appreciated.Sreid, the bushings are not sintered although sintered and oilite bearings would have been a better design as the contained oil usually migrates to the areas where it is most needed.I suppose I really needed to hear what byrdj and jspisich both said in essence that the unit could have been designed better.I am new to the company(Global brewery) and am just getting familiar with the plant (high speed canning lines).Ten million pounds was spent last year on the back end of the line, packaging and palletizing.Most of that investment was spent on Italian made and installed machinery.Apparently the firm involved has undergone phenominal expansion from more or less a cottage industry to a market leader in packaging solutions.Some of their concepts are really innovative and their machinery is aesthetically beautiful but their nuts and bolt engineering seems to be somewhat lacking.Take the bushings, housings and shafting.Its an end of line(food standard)application so lubrication was going to be an issue,even food standard grease was frowned upon in case a tiny smear fell and contaminated the product.Phosphor bronze sleeves pressed in to butter soft aluminium just is not up to the task and this is one of about a dozen design flaws so I will no doubt be posting a few more threads.I will put forward your views to our Eng. team.It was great hearing from you,I think i would do well to stick around.
Thanks,
Jamie.

 

[sreid]

Ok, then how about Polymer Bushings such as

http://www.hefusa.net/high-load-polymer-bushings.htm

 

[ukjay]

The Igus and polymer bushings look a promising alternative. I am toying with the idea of hardening the guide shafts and rack bars and putting a thin nitride coating on them which would allow me to install linear ball bushings.We have discussed having a model made (1 off) and just constantly cycling it to see how we get on.I need to get some lube on it though even just the slightest oil film will no doubt stave off scuffing and spalling till the parts have earned their supper.Load figures are almost negligable its just the sheer number of cycles the head does,day and night six days a week.Anyway thanks again,
Jamie.

 

[byrdj]

I had one experance with linear bearings and the failure occured on the shaft that the bearings rolled. the machine vibration caused the balls to indent the guide which was harden to C65 about 1/8" deep. thus the shafts you plan on harden will become an expendable. these were immersed in a continous feed of turbine oil. I like the ideal of a polymer that would require no lubrication if at all possible. and it elimantes the contamination from bronze dust.

 

[sreid]

Two more similar thoughts for you; Rulon and UHMW PE.

http://www.sdplastics.com/rulon.html

http://www.sdplastics.com/uhmw.html

 

[DesignerGuy16]

Given low loads and high cycle, the IGUS bearings should do the job. Stay away from their A200 "food grade" material.

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

 

[ukjay]

Hey Guys,
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

 

[unclesyd]

Could you use this type of Spieth guide bushing for your application. We used a lot of these for guides in D.I. and Al housings. The only problem with this approach it is not the cheapest way go, but they work.

http://www.ame.com/fileadmin/user_u.../AME_spieth_guidebushings_FDK-FDL-FSK-FSL.pdf

 

[gruntguru]

I was starting to think about an alternative axial location when you mentioned "top-hat". Possible even the bronze material will work if you can locate it axially with collars or similar, rather than relying solely on interference in the bore.