Backhoe swing pins

[Welder Dave]

Last year I repaired my swing post pins and bushings. I used spring tension bushings because the holes weren't perfectly round. I had a hyd. shop make new 2 1/2" pins. Initially everything was working good. A little while ago I noticed what looked like powdered metal around the top swing pin. Further investigation revealed that the bushing had dropped down about a 1/4". It's a 2" bushing so still had 1 3/4" in the bore. My plan was to pull the pin and pull the bushing back up into place. Then I would add some large spacer washers to prevent the pin from dropping again. I managed to get the pin out and it looked like the bushing may have turned about a 1/4" of an inch. What was more disturbing was the back side of the pin (facing the hoe bucket) was severely worn about 3/16" deep. Front side of pin had some wear/gouging but not too bad. The bushing was gouged and rough but still had a decent thickness.
The bushings are spring steel designed to be the wearing part for cheaper repairs but in this case the pin was severely worn. I'm wondering if the pin material was too soft under the chrome or if once the chrome wore it acted like an abrasive on the pin? I think the pin may have been made from a hyd. cylinder rod. One of the original pins isn't worn too bad so I can reuse it but it's puzzling since the top and bottom pins have to be in line with each other. I'll need to remove the bottom pin as well to replace the bushing and hoping it isn't also severely worn. I don't see any of the powdered metal around it so crossing my fingers.
I could possibly see more wear on the pin near the top of the bushing that slid down but the groove is the full length of the bushing. Has anyone ever seen a pin wear and leave a fine powder behind? I used dry graphite for lube but I think it would have worn off pretty quick.

 

[Cat977]

I'm not familiar with backhoes, but I would think a grease fitting and Molybdenum grease would be best for a lubricating. A grease grove in the bushing opposite from the loading side could help
"(Stop 1/8" from the edge so the grease doesn't just squirt out the side)". A hard pin and hard bushing replaced as a set makes some sense since it doesn't rotate very far and it carries very high loads. In a place of moderate loads a soft replaceable bushing is good, it gets its strength from what its pressed into.
Chrome is not very thick, but it is hard, so maybe the pin is flexing. A stock pin is probably high grade steel, case hardened.

 

[sawmilleng]

What Cat977 sez. I read into what you said about dry graphite as a lube as a not-so-frequent lubing. If the friction between the pin and bushing got high enough the chrome would take a beating and soon flake off, exposing the softer steel underneath. And the crushed-up chrome would make a great grinding compound. What you describe is pretty indicative of not much lubricant and lots of loading working to tear up both the pin and bushing. The fact that the bushing turned a little adds to the conclusion that there was LOTS of friction between the pin and bushing.

The same setup might work just fine with grease grooves ground into the bushing or the pin (where it contacts the bushing), if the pin is bolted into place so it cannot turn. The grooves need to be put on the load side of the pin or bushing. Having the grease fitting in the end of the pin may make it easier to get at and avoid having to drill the backhoe frame. Also, you don't have to worry about the bushing turning and plugging off the grease opening into the inside of the bushing.

The comment about moly grease is good, too. I use a very thick moly grease (NLGA 3 moly) that tends to stay in the joint longer.

To get an idea of the loads the swing bushings see, imagine the boom stretched straight out- it would be about 16 feet long. The vertical distance between the two swing pins is about 2'. If you put 100 lbs on the end of the boom, the two swing pins will see an 800 lb load at their bushings. So the load multiplication on these two little pins is massive when digging. If you have the boom pulled in half way, say to 8', and are reefing on a huge stump with everything the machine has, (max. breakout force) of say, 15,000 lbs, those pins are resisting 60,000 lbs through their bearings. So a little lube will go a long way.

Good luck and keep us posted!!

Jon.

 

[Welder Dave]

Got the bottom pin out and it and the bushing are still in pretty good shape. The bushing still feels smooth. I'm not going to worry about the bottom pivot right now. I'll put a bunch of grease on them when I put it back together. The top is another story. I think the bushing may have twisted a bit if that's even possible. The top 1/4 inch towards the hoe bucket is worn about 1/8 of inch and the bushing had turned a bit. Bore for the bushing is still fairly round on the side facing the seat. Taking the original Cat bushing, it fit in the bottom of the hole. It is just mild steel that the bushing goes into and will definately need some welding to repair the bore. Short of line boring, I'm wondering if the hole saw idea might work or if there is another DIY way to get a round hole other than using a die grinder and then a flap disc?

 

[Willie B]

Do you really want to do this twice?
Get a competent line borer to weld the bore & rebore it to specified dimension, or bore it larger & machine a new bushing to match the bore.
You have a problematic hoe. Look at other newer hoes. The engineering trend is bigger steel & more distance between top & bottom pins. It looks light duty design, you'll have to do EVERYTHING right to make it functional.
Then, you will have to grease 3 times a day to preserve it.

 

[Welder Dave]

I wouldn't say it's a light duty designed hoe. I think there were some things they could have done differently, like not putting a 1/8" NPT grease fitting in 1" steel at the narrowest point around the welded in bushing. Had that grease fitting not been there, I don't think the top swing mount would have ripped right off. 1/8" NPT needs almost a 3/8" hole. Hoping to start on welding up the bore in the next few days. I went to Metal Supermarkets to see if they had a small piece of copper to use as a backing for welding the bottom edge of the bore. They had a piece, 5" x 4" x 1/4". Cost $40! Got a 4" x 4" x 1/4" piece of aluminum for about $7. Had no idea copper was that pricey. Welding the bore up is the 1st step. Line boring is not an option. I'm going to very carefully see if I can set my drill press up to be square and level with the bushing. If I can I think I'd try using a 2 7/8" hole hole saw with a guide tacked on to get a fairly round hole a little undersize. Then I'd use a 3" flap wheel to carefully finish the hole. My thinking is a 3" flap wheel would take off the high spots 1st and leave a rounder hole. I could use a spring tension bushing if the hole wasn't perfectly round for the standard bushing. First I need to weld the bore up and see how that turns out. Then I can decide on how to finish the bore.