40+ years ago it was pounded into me at my father's knee that fast travel in reverse wore the pins and bushings in a loader's (or dozer's) tracks faster than in forward. The explanation involved the bunching up of the track in front of the drive sprocket when the slack appeared there. I was told you didn't dare travel at full speed in reverse or the track life would be severely shortened. In all my travels since then and browsing on the internet I haven't heard that since. Could there be any truth to this? The machines in question were 955H/955L loaders.
Yup (from an old Dresser catalog I found)
We limit our dozers to 2nd speed in reverse for that reason.
Huntoon, can you print page 17, please? Thanks for what you've already put up.
http://huntoon.us/wearing_characteristicspg17.png
Thank you! I'm surprised there isn't more discussion of this.
Yair . . . This has been talked about for years. It stacks up on paper but in the real world? I dunno. When pulling and raking a tractor is mostly in second and consiquently when walking into the wind (between jobs) I would have no issues with turning them around and walk the buggers backwards . . . it's much easier on the eyes. Any one have theorys on how it stacks up with the high-tracks? In theory if you moved the sprocket forward so it was centred over the trackframe the drive characteristics would be the same in forward and reverse . . . or is there something I am missing? Cheers.
Perhaps more even wear on the track chain and sprocket but more you wear out two idlers instead of one.
What you are missing is that when traveling in forward when the bushing moves into the sprocket the pin turns inside the bushing. The trailing holds the same position all around the sprocket. When traveling in reverse the bushing rotates into the sprocket teeth and wears against the back edge of the sprocket and forward half of the bushing. That's why when you look at a worn bushing, the most wear is always on the leading side. System 1 tracks do away with the issue by having rotating bushings and multi piece links.
Reading over the information here, it seems that the 30 degree rotation or sliding of the bushing against the sprocket is the main cause of wear when traveling in reverse. I'm having trouble picturing this as the top and bottom relationship between the sprocket and bushings are the same to me. Can someone explain this in simple terms so I can try to understand the difference. I'm thinking that in a foward direction the bushing is sliding away from the root and in reverse the bushing is sliding towards the sprocket root.
The issue is that the bushing is locked to the small end of the track link and cannot rotate. The pin is inside the bushing and locked to the big end of the track link. The pin can turn inside the bushing so going forward the bushing locks into the sprocket tooth and maintains that position all the way out of the sprocket. The pin end of the link rotates turning the pin inside the bushing. When in reverse the bushing enters the top tooth of the sprocket and as it transitions from straight to go around the curve, the trailing part of the link that the bushing is locked to has to rotate with the bushing locked to it. It sounds confusing but if you walk next to a crawler and watch it moving in reverse it will become clear.
Thanks! Those words made it click for me. In foward, the track is coming off of the sprocket in a straight line.
Uggg....I'm staring at page 17 again and seeing the same thing in foward and reverse. I guess I will just have to accept what everyone is saying.
John described it well. The pin rotates inside the bushing when going forward. The bushing is fixed to the end of the track link that lifts to follow the sprocket teeth around, and once it contacts the tooth, it is locked in and does not turn in the tooth. The other end of that link is fixed to the pin, and the pin rotates inside of the bushing on the next link. With sealed and lubricated tracks, this joint is oil filled, and has hardly any wear, as long as the seals are good and the oil is still in there. When the oil leaks out, you get a dry pin, and it wears out from the inside and causes the tracks to stretch. The bushing rotates in the segment tooth when backing up. Keep in mind it does not rotate all the way around, it only turns the 30 degrees or so one time, when the track transitions from flat to the curve of the sprocket, but that one small turn, repeated thousands of times over, wears the bushings and the sprocket teeth like a grinding wheel. Add some kind of abrasive dirt to the mix, and it accelerates beyond belief. Hopefully this helps with the understanding of the reason for the faster wear in reverse. .
Thanks JD, I understand everything you and John wrote. What I'm not following is in a foward direction the rails/bushings are making the same movements coming off of the sprocket down at 6 o'clock, but in a root to tooth direction. Does the weight of the machine keep the bushing planted in the root better instead of allowing it to ride towards the thin part of the teeth as it would in reverse? With everything being equal on paper, I'm trying to understand what is different between the 2 directions.
The reverse drive side external bushing wear compared to forward drive side wear is easier to understand if you go and look at the actual track chain design and construction. Conversely if you were to install the track around the other way ( backwards) then you would experience accelerated levels of forward drive side wear to the external bushing. This is why the track chain / track shoe mounting holes are universally designed so the shoes can only be fitted one way.
I think the point might be when the bushing is under LOAD and sliding instead of just going for a ride.
Yair . . . Like Old and Worn I too have trouble with the concept . . . I accept it for what it is. One thing that is obvious though is the fact that in reverse (especialy if the tracks are a bit slack) loose track is being fed onto the ground which on some tractors can "bunch" between the sprocket and the back roller and indeed between rollers when traversing across logs and rocks. This can cause some accelerated wear on the rails and roller flanges. Cheers.
Simplest way I can explain itis, picture a couple of links laid on the floor. Pick one up, the bushing rotates relative to the floor. Now lay it flat, pick the other end up, the pin turns in the bushing. Now imagine the sprocket in there.
For wear to occur you need 3 factors, Contact (external bushing to sprocket tooth),Load and Relative motion. So in reverse we have contact, relative motion ( external bushing to sprocket tooth) but low load so wear is also low. In forward we have contact, load but no relative motion ( bushing to tooth) so low wear. Put the track on backwards and you have all 3 factors and high wear.
Maybe this diagram will help. From it can see that the pin is pressed into the "wide" end of the track links and the bushing is pressed into the "narrow" end. Imagine lifting the links #7 & 8 at the LH side while keeping the rest of the chain fixed and the pin #1 on the LH side will rotate with those links while the bush #3 remains fixed in place. That's what happens when the track moves in the forward direction, the bush stays (relatively speaking) fixed with respect to the sprocket because it is not being forced to rotate. However lift the links #9 & 10 at the RH end of the assembly and the bush #3 is forced to rotate with them because it's pressed into them. However the pin #1 will not move because it's pressed into the links #5 & 6 and we are not lifting them. That's what happens when the tractor moves in reverse so the bushing is being forced to rotate with respect to the sprocket teeth and that's what causes the higher wear in reverse because of the forced rotation of the bush with respect to the sprocket teeth as it enters and leaves the sprocket.
Yair . . . gotcha Nige and ih100's analogy was good too. I understand that, what I have not been able to pin down is how much difference it makes in the real world. In our pulling application the tractors could do ten or fifteen miles around a paddock with just a few hundred yards of backing up and we certainly didn't get big hours on the walking gear. As I said, I accept and understand the factors involved, but the suckers got to back up sometimes, it's just a part of doing business. I certainly agree with avoiding excessive speed in reverse . . . and forward for that matter. I watched a high track eight working with a slack the other day and it looked bloody horrible flapping around. In a different life I maintained four nine G's pushing sand. We didn't bother turning and ran the bushes to destruction--mostly just ran out of adjustment. They were'nt SALT of course and the pieces always fell out on the forward drive side of the bush i.e . . . any wear occasioned by reverse was pretty much irelivent. We were lucky if we got twelve hundred hours, half that on occasion when working in tidal slop. Just saying as I saw it back aways, SALT tracks and the new generation walking gear must be a whole 'nuther ball game. Cheers.
High Tracks the wear can be significantly greater due to the change in angle of the bust contact to sprocket segment. This is what makes System One Undercarriage clever. the bush rotates separate to the chain links.
That is not what I have experienced. In my experience, the rails scallop on the high drives because they run around 2 idlers, but other than that, I have had better life out of the newer gear than I ever did with the older oval track machines. Running D-8H and K series, 3500 hours was a good life out of an undercarriage, in the conditions I run in. With D-8R and T series, I am seeing over 5,000 hours on rails and sprockets, and near double that on the idlers and rollers. The expensive part of the undercarriage is the track frames, bogies, and such. With so many more moving parts than a rigid oval track machine, the cost is up there, but those typically only need work every 2 or 3 sets of rails. I believe the production gained far outweighs the cost of those parts.
The OP post is about Foward and Reverse......not High Track Vs Flat Track!
Yair . . . CAT793. Point taken but I think the comments are constructive and I for one have often wondered how the geometry of the Hightrack may affect track wear when you consider that the sprocket is mostly working above the slop. I had thought the direction of travel would be less of an issue with the hightrack design. Cheers.
I 100% agree with everything JDOFMEMI said! I was just clarifying my comments were in relation to the OP. If we are now changing the Topic I would add that the top Carraier Roller on the 7PZ D11R has helped heaps in the scolloping of the rails. And the 3x bolt roller cap were also a big step in longevity.And as suggested locking out 3rd pays big benefits. Is this a similar situation to the smaller high track dozers . I only get to play on D11 and D10 size TTT.
It would seem as if every tracked trencher I have seen is built backwards then. The sprockets are always on the front of the machine, so when digging the chain goes around the sprocket top to bottom. Is this an over site in all the designs or are they built differently.
Not having seen the design of the track link assembly it's hard to comment, but what's the betting the links are designed to run backwards - in other words assembled the opposite way round to a "conventional" tractor ..?
Thanks for the added explanations and pics. I can clearly see the bushing rotating only in reverse now. I was trying to understand this by looking at a small chain and sprocket but it's not built like a track as all of the bushings can rotate freely.
Regarding the trenchers, I think they drive from the front like they do due to space limitations. No room on the digging end for the final drives. I think that since a trencher travels much less than a dozer, that the wear from going the wrong way is less of an issue. I have not seen it all by any means, but I have never seen a trencher with the bushings worn through like a dozer. I am sure there are some somewhere though, but it would appear to me to be much less of a concern on a trencher. I have never paid attention to one to notice if they rund the chain backwards like Nige says. I will look next time I am around one now since you have gotten my curiosity up.
As someone pointed out a while ago the new Cat System One undercarriage has fully rotating bushings like a roller chain and so should not suffer from the "reverse wear" phenomenon. However I hear that Sys One has been tried on larger than D8 size and isn't up to the mark as yet ............ right now we're trying some new link groups on our D10's that have a deeper hardening in an attempt to combat rail wear.
Nige What kind of life do you see on a set of rails where you are at? Are you in conditions that cause the wear to be greater than normal?
Our major wear issue here on our tractors (D9-D11) is abrasion. The components that really suffer are grousers and sprocket segments. Grousers (and we are using Super Extreme Service - the biggest nastiest thickest grousers available) can go as little as 500 hours if the tractors is on abrasive ground all the time. Sprocket segments will go about 1500-2000 hours in the same conditions. By rails I assume you mean the link assembly. We don't bother turning pins & bushes, just run them to destruction. Using SALT links with PPR pins we are getting around 8-10,000 hours on a set of links with God knows how many shoe changes in the interim ............ I've got D10 tractors up to 12,000 hours still on original track rollers (40-50% wear) and idlers (45% worn), but then again we lowboy them from one side of the site to the other (about 2-4 miles one way depending) D9 track rollers don't do so well, around 5000 hours to re-shelling. Idlers hold up well though, much the same as the larger tractors.
Nige Thanks for the info. Sounds like some very abrasive material. I am amazed by the grouser life at only 500 hrs. On the other hand, I am equally amazed in the other direction at the 8 to 10K link and bushing life. Sounds like a trade off. I am assuming this is mostly doing duty pushing the truck dump, and maybe shovel clean up. Short travel with a high load factor. What I am scratching my head about is the segment life being so low. I am used to segments usually lasting at least half of the bushing life, and in many of my machines lasting the full bushing life. For contrast, my D-8T got 5500 hrs on links and bushings the first time around, and we put the original pads on the new links. It now has about 8800 hrs, and needs regrousered I bet you go through track pad bolts by the pallet load.
S'funny you should say that. We buy them by the pallet ......!!! Our target is to get 50% re-use on track bolts, but it's usually less than that. When re-shoeing we scrap the bolts furthest away from the grouser. The bolts removed from the hole nearest to the grouser are re-used in the hole furthest away so as to wear them out, and then we use new bolts in the holes next to the grouser - if that makes sense. Nut re-use we are around 95%+. On 1 or 2 sets of shoes I'm going to try welding a short length of square grouser bar (probably about 3/4" square and maybe 9" long) on the trailing end of the shoe right next to the 2 bolts that get the most wear to see if the bar protects the bolt heads sufficiently for them to be re-used when changing shoes. It could reduce my bolts costs significantly and the bar should outlast the shoes.
Nige I worked for a man that ran a gravel pit and he had some old 8's that pulled pans. He also had a few that were uped to pile gravel and he had some factory grousers that had a set up like our thinking about. I dont remember where he had gotten them but he bought a truck load they were all new old stock. He also rotated the bolts like you are doing. But the twist on that is he had us run a bead right in the top center of each bolt. Just a big dob like bird crap, just keep it off the sides of the bolts.