Stress points on backhoe pins?

[Welder Dave]

I'm putting new swing tower bushings in and using split tension bushings to have a tighter fit in the bore. I want to put the split opposite of where the highest force on the pins is. The top pin I'm sure is pulling on the bucket side of the bushing. The bottom pin I'm not sure if it's also pulling on the bucket side when digging or compressing on the machine side. Normally I would think it's twisting and the top is pulling and the bottom is compressing but not sure. Maybe I'm over thinking it but figured someone on here could explain it.

 

[Tinkerer]

You are not overthinking it Dave.
You have a good understanding of the forces applied to the pins and bushings.
I think the force on the top pin is normally on the outer (rear) side of the top pin when the boom is straight out from the rear of the tractor.
And the force is pushing against the inner (front) portion of the lower pin at the same time.
Of course the direction (location) of force will change when the boom rotates.
That is my understanding of it IMHO.

 

[hosspuller]

I think the forces change with the operation. (lifting the boom vs digging) How about placing the split to the side ? Seems like most of the time you're directly on the machine centerline.

 

[Tinkerer]

I dunno Hoss. What about putting the top bushing with the split facing forward and the lower one with the split facing rearward. Then wouldn't the force of the weight avoid the splits even when the boom is swung to extreme left or right ?

 

[hosspuller]

Tinkerer … The force direction change was the reason I replaced the boom pins & bushings. All was okay until I applied force to dig. Then the boom lifted at the pivot with a clunk.
We may be overthinking the whole issue since the split bushing Welder Dave is using has interlocking fingers at the split to support the pin. It's not as if the split is a straight line. Dave should ask Connex. They have the most experience with the bushings.

A point I read on their site... They intend the spring bushing to be a "wear" part. Taking a swing tower apart to replace bushings sounds like a big ordeal.

 

[Welder Dave]

They say to put the split opposite the highest force, usually 90 degs. but I dig a lot from the side (digging sand from a pit). I was thinking to put the top bushing at 11 o'clock looking down from behind the machine (backhoe side). This is where the bore is slightly larger but I figure would give the most surface area for the bushing to expand against the housing. The bottom pin forces have me confused. The bottom pin had deep grooves from the bushing it in and I didn't note what side they were the worst on. Top pin had some wear but not real bad. When I had it apart to weld, the pins were still in good shape so I never thought to check when I pulled the pins out this time. I'm not sure if when digging both pins are pulling back like on a drawbar or the force is opposite like a door hinge. Not even sure if it makes a lot of difference where the split is.

Taking the hoe off the swing post went pretty smoothly once I thought about how to do it. Putting it back on may be a different story though lining up new pins and bushings.

 

[Welder Dave]

The 1st pic. from this recent post is what has me wondering about the forces on the bottom pins. With the stick coming in while digging it would seem that it pulls backwards on the bottom pin (same as the top) but when the hoe is up would put pressure on the front side of the pin closest to the machine. I think the most force would be when digging???

https://www.heavyequipmentforums.com/threads/need-to-find-a-used-reman-cylinder-for-cat-426.78746/

 

[Billrog]

Dave if you keep the machine well greased with the new bushings I'm sure the stress points are of no concern. My machine has 6700 hrs. on it now & no play in any pin. I've only repined and bushed 1 machine at 9,000 hrs and it was one I bought with 8,000 hrs with some play in the pins.

 

[Welder Dave]

Apparently spring tension bushings don't normally require lubrication but lubrication can reduce noise and increase life. They make special versions with a groove and grease holes. I also read that graphite will help and won't attract dirt so I'm going to spray dry graphite on the pins and bushings. When I did the steering clutches the shop recommended using graphite over grease on the ball pivots because it won't attract grit like grease will.

 

[Welder Dave]

Just to give an update on the new pins and bushings. I put the bushings in with both splits at the back because it appears the most stress is when digging with the boom down. Both pins are pulled toward the bucket. I also used Loctite 660 because it supposed to seal gaps up to .020" and help repair less than perfect bores. I figure the combination of the Loctite and the spring tension bushings should keep the bushings in place. I think any out of roundness is less than .010" except for one small section on the top pin.

Bushings went in easy by using a flat piece of steel on top and good smack with a hammer. Trying to just hammer them in didn't work because it was an uneven load. Even with the plate they went in a little crooked but straightened out after they got past the chamfer on the end.

Lining the pins up took some imagination and experimenting doing it by myself. Using an engine crane, hyd. jack, com-a-long, and ratchet straps I was able to get the pins lined up. The boom cylinder had settled about an inch but I found by lifting on the boom cylinder and then pushing the valve for the cylinder to lower, the boom cylinder extended and got close to lined up. Jacking from the bottom and a ratchet strap helped to get it real close and I was pleasantly surprised the top pin popped right in. I put a bolt in the mounting hole on the pin so it wouldn't go all the way down. The bottom pin was tricky to get lined up. It had to move back about 3/8 of an inch and also has big washers on each side of the ear. With a com-a-long attached pulling the boom back, I found that jacking the bottom of the swing post with just the right pressure caused the swing frame to move closer into position. I had to give it a few hits with a hammer to get it even closer. I jacked it up so I could align the bottom washer. Then I let it down a bit so I could align the top washer. When it was close I was able to tap the pin in and the pressure held it place so I could put the holding bolt through. Then I took the bolt out of the top pin and tapped it down into position and put the holding bolt in it. I used some dry graphite spray on the pins. I was so relieved I got it back together with my crude set up. Hopefully I'll never have to take it apart again.