Recommended Linear Bearing, heavy load, poor rail surface

[T1357]

I am needing to replace bearings on a seamer. The sliding portion hangs on 4 bronze open bushings on two round steel rails, rail diameter 2.5". Speed is 2-3 feet per second, load is over a thousand pounds, linear distance traveled is 1-1.4 million feet a year. The last set of bushings lasted about a year (and lost over 1/16" of thickness from the loaded sides), but we don't even have records of what type of bronze was used. I know that my biggest problem is that the rails are old and damaged with occasional divots, grooves, etc. However, replacing the rails is not an option I have. I don't need the bushings to be tight fitting, just extra durable. Would ball bearings hold up better to the uneven surface, or would they just not be able to take that many feet of travel? Should I just go with an aluminum bronze like C954 and hope it lasts over a year? Would a PTFE bushing handle that much load?

(This really isn't my area of expertise, just filling in for an engineer that retired recently)

Thanks!

 

[T1357]

I did find out that the current bushings are C932 bronze, and haven't been greased properly either. Still have no idea what material the original design used.

 

[Tmoose]

Please post a picture showing how much real estate is available for fitted trolley wheels etc .

What is the travel in operation? Simply providing lubrication and sliding covers or accordion boots might be expected to increase life 100s of X.

How long are the bushings ?
The simplified projected loading in psi is often an important part of first cut calculations. < 100 psi < for instance

These guys say the C932 is "light duty."

https://www.dura-barms.com/bronze/leaded-tin-bronze/c93200.cfm

Usually leaded bronzes are claimed to be the best candidates when lubrication is scanty.
Conversely the fancier aluminum bronzes etc NEED effective lubrication.

There are some fancy composite bushings claiming excellent wear with NO lubrication. In fact greasing them shortens life by 90%, according the the brochure and the tech support folks as well.

 

[SwinnyGG]

If the shafts are torn up, ball bearings will die a quick death.

Tmoose is right with recommending that you focus on lubrication and better bushing materials.

 

[Compositepro]

If the rails have dings, then new ones will probably soon have them too. It seems to me that using some type of wheel with sealed bearings to ride the rails is a better approach.

https://www.bwc.com/

 

[btrueblood]

Using aluminum bronze without lube is going to rapidly wear both the bushing and whatever it rubs against. Can you use an oil-impregnated PM-bronze bushing?

 

[T1357]

Sorry, been out for couple of days. Thanks very much for the input. I'll try to put up a picture but I can't get a good one. The open side of the bushings is directly on top, where the rails mount on 1" thick bar. So the open top side cuts down a lot of the load bearing capacity of the bushing, with the load applied on each side of center. Above the load bearing areas of the rails I have about 1.5 inches of clearance. The current setup uses an inch of that space. The travel distance is 12 feet for each bushing, total rail length of 20 feet. I think we could do better on lubing the bearings, the last ones had a very shallow grease channel and the grease wasn't getting all the way around. The bushings are 6 inches long, I'm estimating 400-600 pounds load on each one based off of the volume of steel supported. I'm leaning towards a harder bushing that is properly greased. My thinking is that the dings on the hardened steel rails are taking chunks out of the soft bronze, and yes I do see large shavings of bronze under the machine. Of course that is only my thinking...

 

[sreid]

My suggestion; Bronze Bushing from McMasters

https://www.mcmaster.com/#2868t274/=1cqtolo

Stone the shafts to get rid of "Proud" defects.

Consider installing rod covers {rubber articulating sleeves."

 

[sreid]

Bellows

https://www.mcmaster.com/#rod-covers/=1cqtrt2

 

[T1357]

Thanks, I think I will stone the shafts. Can't really find an effective way to cover the shafts though. Two problems: first the shafts are 20 feet long, second they are attached along the entire upper side so any covers would have to be "C" shaped. I think I'll give the heavier-load bronze a try for right now, and make sure that these are made right so that they can be greased properly.