Help needed on the "bushing-housing fit on an earthmoving machine" 3

[serdart]

hi all,

I'm supposed to find out the required fit for the bushing-housing couple of an excavator attachment (boom-arm-bucket). But I dont know what the design methodology should be. There is not any direct lateral force on the attachment that forces the bushings to get out of its housing. There is a lateral force on the tip of the bucket, but there isnt a direct analytical relation between bucket lateral force and axial forces on bushings. This might happen due to the torsion of the boom and distortion of the housing, but this approach is hard to implement(FEA requires time..) WHATS UR RECOMMENDATION?

 

[MikeHalloran]

Consider also the torque applied to the bushing in normal operation by joint rotation when the pin hasn't been greased since it was new, and the machine has been left out in the rain, or submerged in who- knows- what repeatedly, and dismantled and reassembled in a dust storm using only hammers as tools.

And when the pin has been assembled like that, and they have to use sledges to drive it out, you don't want the bushing to slide in the housing.

Don't take my word for it. Go out in the shop and ask an Old Fart.



Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

In response to the old fart call.

If the bore is too worn it can be welded and remachined.

http://www.connexusa.com/HTML/principle.html

http://www.rollpin.com/ProductInformation/TensionBushings/index.ht

ml

 

[diskullman]

These pins and bushings are usually tapered.

Russell Giuliano

 

[unclesyd]

It just struck me that you may talking about a bronze bearing/bushing. Normally a bronze bearing is installed with a shrink fit to prevent movement. This works quite well until the bearing gets hot or little worn. If the bearing thins out it will move when a load is applied so you get the bearing working out of position.
Look over this site to get some idea what it takes to install a bearing such as yours.

http://www.beemerprecision.com/bearingCalc.asp

http://www.beemerprecision.com/PDFs/castBronze_designData.pdf

http://www.beemerprecision.com/index.asp

 

[serdart]

Thanks for all reps,

Mine is not a bronze one, it's a hardened bushing from 16MnCr5. In fact, I've moved on a little bit (with assumptions of course) and am required the elasticity modulus of the bushing. However, it is a cementited bushing that has a hard surfece and softer core. Any method for calculating an equivalent "E" value to this non-homogenous bushing?

 

[EdDanzer]

Logging and construction equipment bushings must be a press fit. .001” press per inch of OD is a number I have used for years. The bore of the bushing will shrink when pressed or shrunk in so the bushing must have running clearance and shrinkage allowance machined in before installing. In many applications we have gone to aluminum bronze instead of hardened steel. If the load spike is high and the pin deforms much it will edge load the hard bushing and cause it to break, then the bushing chunks will grind the pin and bushing bore up. If the housing material is too soft, the hard bushing will cold form into the housing and then the bushing will either spin of break up.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[serdart]

So, no problem solution or numbers? these kind of applications, I guess, are more likely to involve engineering sense and experience. Maybe I can derive one, based upon these idea..

 

[serdart]

And about the deformation of members; Is this approach logical: I think that housing should not be plastically deformed even if it is loaded. Because in the plastic deformation case, when the bushing is desired to be replaced, the new one will be fit into a larger hole meaning a looser fit. And also housing loses its circular shape because of similar directions of loading every time. But loads on these bushings are extremely high, so in order not to exceed elastic limit, initial pressing pressure becomes low. Am I right in this dilemma??

 

[EdDanzer]

With .001” per inch of bore and a bushing with a wall thickness of 20% of the bore or less the bushing pressure should not deform the bore when installing a hard bushing in a steel housing. One thing to remember is mild steel has a 36,000 psi yield, the bushing preloads this some and the work induced loads are additive to the preload so the actual capacity of the mild steel housing is not much more than the film strength of the lubricant.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com