Excavator arm pin and bushings problem

Hi,

I don't post here often but I'm having a bit of trouble with my mini excavator. It's a Bobcat E60 with a long arm. I bought the machine almost 4 years ago and when I got it, I thought it needed new pin and bushings for the arm to boom joint. So I ordered them from Bobcat and when I got ready to install them I slipped the brand new bushings over the brand new pin and there was a fair amount of slop. I'm no mechanic, but I usually work on my own equipment when I can and it seemed way too loose to me. So I contacted the dealer to make sure I hadn't gotten improperly sized parts, and they sent me tech drawings of the pin and bushings. I measured and they were within tolerance. So I installed them and it seemed a little tighter but didn't take long to get loose again (yes I do use grease ).

I find it hard to believe that Bobcat designs these with that much play. I'm thinking that if I can get a new pin and/or bushings with tighter tolerance that it will stay tight longer. Does that make sense or is it wishful thinking on my part? I've attached the drawing from Bobcat. Seems like 0.28-0.35 mm clearance is a lot for something of this scale.

I would appreciate any input.
Thanks

The story isn't complete without knowing the diameter of the housing for the bushing. Whatever press you have on the housing/bushing fit will reduce the id of the bushing somewhat, depending on the amount of material surrounding the bushing.

I guess I'm not exactly sure what you're asking. The outer diameter of the boss that the bushings press into? Or the inner diameter? I installed the bushings after having them in the freezer and they went in nice and tight. But the pin still rattled around when I did a test fit. I took a video of it https://photos.app.goo.gl/X9pwL3xAe3HDXtHc7

Bushing to pin fit should be a little loose to allow for movement and greasing. Are you perhaps seeing loose pin to boom fit. Mini-ex's aren't exactly built tight from the beginning.

I understand that it can't be super tight, but there are grooves in the bushings for greasing. Not sure if the shared video is working but it shows the loose fit between the installed bushings and the pin. There may also be a small amount of space between the pin and boom bosses but the pin doesn't rotate in the boom so it won't wear there (at least not at the same rate). I could try to measure the amount of sideways movement at the bottom end of the arm. Any idea what would be considered too much movement?

I have more experience with backhoes. I'd say .002" clearance affords space for grease. I'm wondering if these things are shipped with mated pairs of pins & bushings. I find pin or bushing diameters are usually nominal. A 2" specification might be 1.979 for bushing 1.977 for pin.

Those I've seen are outrageously hard bushings. Pressing the bushing in does not change the ID in any measurable way. Where in VT are you?

Sticking the one end through and wiggling it just says it has clearance. If you were to push up and down on both sides at the same time, the visual would be much less dramatic. For side to side clearance, you could possibly get some bucket shims the appropriate diameter to take up some of the engineered slop out of it. According to the provided drawings, theoretically, you have no slop. But it shows desired at 50mm +/- .23-.25mm on each. So let's say for argument sake, your pin is 49.75mm and your bushing ID is 50.25mm. That would be a tolerance stack of .5mm You're still in limits for new, but have a clearance that might be on the outer edge.
Hope that helps. I don't think you really have an issue to be concerned with.

Willie B said:

I have more experience with backhoes. I'd say .002" clearance affords space for grease. I'm wondering if these things are shipped with mated pairs of pins & bushings. I find pin or bushing diameters are usually nominal. A 2" specification might be 1.979 for bushing 1.977 for pin.

Those I've seen are outrageously hard bushings. Pressing the bushing in does not change the ID in any measurable way. Where in VT are you?

Click to expand...

Hi Willie, thanks for the input. I'm in Chester. We've chatted a bit on the WeldingWeb. My screen name is ArcTan over there. 0.002" is a little over a tenth of the amount of clearance that I measured with the new pin and bushings. I'm sure if it was that tight I wouldn't be complaining about it! I realize the sizes are nominal, makes sense that the pin has to be slightly under the nominal size, and bushings slightly over.

funwithfuel said:

Sticking the one end through and wiggling it just says it has clearance. If you were to push up and down on both sides at the same time, the visual would be much less dramatic. For side to side clearance, you could possibly get some bucket shims the appropriate diameter to take up some of the engineered slop out of it. According to the provided drawings, theoretically, you have no slop. But it shows desired at 50mm +/- .23-.25mm on each. So let's say for argument sake, your pin is 49.75mm and your bushing ID is 50.25mm. That would be a tolerance stack of .5mm You're still in limits for new, but have a clearance that might be on the outer edge.
Hope that helps. I don't think you really have an issue to be concerned with.

Click to expand...

Yes, I understand the concept of the clearance. The side to side movement I was referring to isn't sliding on the pin side to side, I have it shimmed tight. The bottom end of the arm will move a little sideways while the boom stays stationary. So it's pivoting sideways on the joint. In the end, I don't really care what the numbers are, I just want to get rid of the slop in the joints that make it loose and hard to operate with precision. I'm know I'm not doing a very good job of describing the problem

materthegreater said:

Hi Willie, thanks for the input. I'm in Chester. We've chatted a bit on the WeldingWeb. My screen name is ArcTan over there. 0.002" is a little over a tenth of the amount of clearance that I measured with the new pin and bushings. I'm sure if it was that tight I wouldn't be complaining about it! I realize the sizes are nominal, makes sense that the pin has to be slightly under the nominal size, and bushings slightly over.

Click to expand...

My point exactly. I don't feel that is acceptable clearance.
Think of an engine bearing on the crankshaft, A loose bearing puts the force on a very small surface. Tighten that bearing, leaving just enough room for oil, the pressure is spread over many times the area. Bearings should ideally leave just enough room for lubricant molecules to create a tight fit.

I'd offer a lathe to tighten the pin size, but I'm not sure without the ability to heat treat, I could improve on the pin.
Israel Mac in Rutland (Mac Steel) has forgotten more than I will ever know. I would have to research the correct steel for pins. they are machined to .0001 correct at raw temper, then heat treated to case harden, then sanded to exact dimension.

A perfect fitting pin works better without heat treating, but wears out sooner.

That's a good idea, I'll have to call them tomorrow after I'm finished plowing. Irv is great to talk to. Hopefully they are able to get tight tolerance round bar. I've heard that 4140 is good for pins, and actually used a piece of it for a pin on my previous machine.

How much snow did you get today?

4140 is the material he sold me for the thumb I built on the Case Backhoe. Ideally, you'd heat treat it when it's close to size. I don't have equipment, or skill to case harden. I think I understand the how.
There is a company, sticks in my mind near Burlington specializes in making heat treated pins & bushings. I can't now remember contact information.
We must have 8". Deer have been visiting. The cedar bushes along the deck are a mess! under the motion sensor lights, they eat most of the night. They ignore the dog, as long as she's inside, and the lights too. They must be some hungry.

Turns out Mac steel doesn't deal with metric.
I'll have to look into the place in Burlington.
Sure is tough on the deer this time of year.

materthegreater said:

Turns out Mac steel doesn't deal with metric.
I'll have to look into the place in Burlington.
Sure is tough on the deer this time of year.

Click to expand...

start with next larger SAE size, cut it to fit the bushing.

You do the same with SAE sizes The bushings (harder than diamond) you might buy are a bit undersized. 2" nominal might be 1.991. A 2" pin won't fit.

I may have found a solution. A guy I know works at LoveJoy machine shop in Springfield and is making an oversized pin for me. They have the ability to test hardness, and to harden things.

Through this process I discovered that one of the boom bosses was somehow worn a bit and was loose on that side as well. So the new pin will be stepped. One end will have a 1-1/2" long shoulder that is a full 2", the remainder of the pin will be 1.970", which is 50.038 mm. Should tighten things up quite nicely.



This is the first iteration of the pin. He turned it to 1.975" which is a little too tight for my liking. Going back for a little more shaving.

I'd run it if you can get it in without hammering. Run it with no load and oil pumped through the zerk for ten minutes and then grease it and go.

That's assuming you're not heat treating it again before installing.

How does grease distribute in this joint? I see no galleys in the pin, are the bushings grooved?

Wouldn't go in without hammering. He took it down a little more and dropped it off this afternoon. Went in nicely now.

Yes, the bushings have pockets in them for grease.

Now that I don't have so much slop in this joint, I can tell which other ones need to be tightened up... Maybe it's time to trade

When you do replace bushings dry ice is available at Maine Oxy something roughly -96F.
They also have liquid nitrogen about -190F. but it is expensive. I'm off some on temperatures I'm working from a poor memory.
Last round of bushings I did, I used canned air upside down to fill a container with liquid. I wasn't real pleased with the results, but I got through it.

Good to know. When I did these bushings I had them in the freezer for quite a while ahead of time, but that only gets down around 0°F.