637E Cat scraper bouncing

[Wolf22]

We operate a 637E (twin power) scraper. We have a problem that when travelling fast the rear end gets a "speed wobble" that becomes a bounce. It is so severe that the rear tyres can be seen leaving the ground if following behind. The problem is even worse when loaded, as the extra weight adds interia making the machine more dificult to control. We have checked the cushion hitch and it seems to be okay with correct acumulator pressures. There are some theories that the tyres are causing the problem, they appear okay externally and are at the correct pressures. We are planning to try new tyres, but are having to wait due to the current tyre supply problems.
I would appriciate suggestions any "scraper gurus" can offer to help us solve this problem. We also have a 651E (single engine) and it has never experienced this problem.

[Countryboy]

Welcome to HEF Wolf22! :drinkup

[Construct'O]

We operate a 637E (twin power) scraper. We have a problem that when travelling fast the rear end gets a "speed wobble" that becomes a bounce. It is so severe that the rear tyres can be seen leaving the ground if following behind. The problem is even worse when loaded, as the extra weight adds interia making the machine more dificult to control. We have checked the cushion hitch and it seems to be okay with correct acumulator pressures. There are some theories that the tyres are causing the problem, they appear okay externally and are at the correct pressures. We are planning to try new tyres, but are having to wait due to the current tyre supply problems.
I would appriciate suggestions any "scraper gurus" can offer to help us solve this problem. We also have a 651E (single engine) and it has never experienced this problem.

With the cushin hitch,and running wide open down the the haul that was normal for the ones i ran and they were 637 C"s !!!!!!!!:usa

[zhkent]

Couple questions.
Is that on a nice smooth haul road?
By wobble do you mean rock side to side, or do they go up at the same time?
Is there enough unevenness in the haul road to get it started?
What gear are you in when it happens?
What is the highest gear you can run in without it happening?
Does it help if you let out of the back throttle?

Empty in neutral you could pan down and pick up the rear end, then put it in 1st or 2nd, and give the rear motor some throttle. A person standing at a safe distance might be able to see if anything was out of round or something.
The rear trans won't shift up above 2nd unless the front trans shifts up, so you won't be able to spin the rears at the speed the wobble happens.

In neutral if you pan down and pick up the rear end then set it back down, you might have a wheel bearing out if a person watching closely can see any movement of the wheel.

Any rear wheel seal leakage? Oil ok in rear diff? On the c models the rear diff also maintains the oil level in the wheels. Don't know if the e model is the same.

Brakes releasing ok?

The frame and axle tube all ok without any breaks or cracks?

What pressure are you running the tires?

How hot are the rear finals (wheels) after running? Can you touch them? Hold your hand on them?

No answers :( only questions :)

[surfer-joe]

Cushion hitches can exasperate problems initially caused by tire problems and it sounds like you might have a rear tire or both that have an internal situation developing. It could be a separation in the carcass or a busted belt. Most of the scraper and other big iron bounce problems I've ever encountered were caused by the tires, with cold bias-ply tires being the worst.

Zhkent has some good ideas at observing the tires. Best way is to follow scraper at speed loaded and unloaded and watch both rear tires carefully for just which side the bounce seems to start and also if any bulging or abnormal flattening occurs.

You can run the tires for a while and then look them over closely. Make sure they are inflated to the proper pressures. Heat will probably be evenly distributed. Look for cracks or large cuts in the tread that might have separated the tread from the belts. Look for bulges on the inside (frame side) that would also indicate separation, although the bulge can be on the outside also. 651's for some reason (39.5R39) always seemed to be on the inside.

Sometimes brand new scraper tires do not get a chance to "cure out." By this I mean that they are so new the rubber compounds used in the tire construction have not set up completely. If run hard for long distances they tend to separate, which is usually indicated by a large bulge on the outside of the tire, but the separation bulge can also be hidden on the inside where you can not see it without taking the wheel down and physically looking inside the thing. If there is a tire shortage at present, this could be part of the problem, tires are coming out of the factories so fast they don't have a chance to sit and cure. Bridgestone had this problem a few years ago in their larger scraper tires and I ran them out of the ones for 651's. Had to buy take-offs and retreads to keep going.

At any rate, looking the tires over and seeing what they do at speed is likely your best bet at solving this problem. There is the unlikely chance that your hitch is malfunctioning, but closely watching it under operation will revel that. Might not hurt to check the accumulator pressure though, also the hydraulic operating pressure, and make sure the linkage is correctly adjusted.

There is one last thing to mention, and that is that in the past, I have noticed some double-barrels get sort of a hop when both engines are running flat out. As I understand it, there is some over-run in the gearing of the rear axle (on Cats) so that they tend to run faster than the front, thus causing the rear tires to flex and start a bounce. This usually happens on a very hard dry surface where the tires do not have an opportunity to slip a bit and even out the speed. Not having a differential in the rear tends to make this problem worse as both tires are shoving hard equally all the time. Slacking off a bit on the rear thottle will stop this, but if your operators are only using one foot on the throttles, this is hard to do. One reason I always used both feet, one on each thottle.

Good Luck!

[Wolf22]

Thanks for the help. I will investigate further and answer the questions as soon as I can.

[chewy]

try reducing rear engine speed. may be over push.

[zhkent]

One more, do both rear wheels spin if you spin under load?

[dirtwhore]

Are they radial or bias-ply tires? Radials will bounce like a bad check all day, no matter what. It wouldnt matter if the hitch was charged or the tire pressures are all the same. Radials dont belong on scrappers or loaders. Save em for the pickups and dump trucks!
Jason

[Deas Plant]

Hi, Wolf22.
Many good thoughts have already been put up by others, especially about damaged/faulty tires. I would add to them that it may help running the rear tires up to about 5 psi lower than the front tires. Care should be taken doing this as too low a pressure may cause damage to the tire that can go unseen until it becoems a 'major'. I have seen twin-power scrapers run with the front tires 2-3 psi over and the rears 2-3 psi under to minimise this damage potential.

Chewy mentioned 'over-push' from running the rear engine too hard. This can also be a factor that might cause this bounce. If this is the case, backing the rear engine off a bit may very well help.

Dirtwhore, people who make 'absolute' statements like the one you made about radial tires on scrapers merely amuse rather than impress me. Have you ever run a mix of scrapers in soft sand, some with cross-ply tires and some with radials? In my experience, the radials will leave the cross-plys for dead in sand. Also, I have seen several companies DownUnder running Michelin tires on their scrapers VERY successfully and I have yet to see a Michelin cross-ply tire.

Just my 0.02.

[Squizzy246B]

Radials dont belong on scrappers or loaders. Save em for the pickups and dump trucks!
Jason

:Banghead :Banghead See...thats what I've been doing wrong all these years:Banghead :Banghead running on sand and gravel in Loaders with those dam radials:eek:

:rolleyes: :rolleyes:

[Dozerboy]

X3 radials have there advantages

[BrianHay]

I spent 4 years on a 37E. The first 3 years it had radial tires on it and I liked them better then the bias ply they were replaced with. For me they worked better then the bias in almost all material except mud and in the mud it wasn't that big of a diference. The radials give a much better ride to.
....Overpush? Never heard of it. Not saying that it's not possible though. But any twin I have ever been on if you drop the front throttle in 8th gear you are going to slow down fairly fast. The rear engine doesn't have near enough power to push as hard as the front unless the front is very sick.
I like the idea of lifting the rear tires off of the ground and spinning them in the air the best. You will find out real fast and easy were the problem is.

[yancy44mag]

are the tires filled with a add if so they will all bounce ben on scrapers all over the world and if they are filled they will bounce hope this helps its just my two bits worth yancy new member

[Countryboy]

Welcome to HEF yancy44mag! :drinkup