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Weak final drive on cat 311B excavator

Discussion in 'Excavators' started by slwbid, Oct 2, 2019.

  1. slwbid

    slwbid Well-Known Member

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    Trenton, ga
    I've owned it for 5 yrs, 2000 hrs. When I bought it, one final drive hydraulic motor was weak. It's still weak. However, very surprising is that I cannot tell that it is one bit worse now than when I bought it.
    The machine has 9500 hours on it.
    I would have thought that if internals of drive were worn, that after 2000 hrs, it would have failed by now. There are no unusual noises coming from it.
    Any ideas what the cause could be?
    It doesnt make a lot of economic sense to purchase a new/rebuilt final drive ($7000), but it runs and operates like a sewing machine except for this one issue and so I will keep using it... but would like to see if there is a lower cost option to this one issue (that is more of annonyance than anything)
    I'm wondering if using heavier hyd oil might help.

    Anyone with any helpful ideas, please advise.
    Thanks
    Stacy Whetzell
     
  2. heymccall

    heymccall Senior Member

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    Further diagnosis is required before blaming a travel motor.
    Next time you're playing operator, travel in a straight line and, while travelling, operate the Stick IN function. Does the travel go straight now?
    Without a serial number, I can only guess that it is a 2 pump system. I guessed at an "8GR" serial number.

    The way it works...
    During travel, each travel motor has a dedicated pump. If a pump setting (calibration) is deficient, one track will be lazier, in both direction.
    But...if you operate another function while travelling, one pump is operating the travel motors, and the other pump operates the additional function (like Stick IN).

    Now, it's actually a tad more complex than that, but not much. That should be enough for you to determine if the pump pressures are equal.
     
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  3. WhyWhyZed

    WhyWhyZed Well-Known Member

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    ^ good way to isolate pump issue from valve/motor issue

    if it isn't a pump, don't forget about the swivel,
    and then control issues -are both travel vales getting full pilot pressure F and R, - is the cab floor clean around the pilot levers, and then are both motors shifting rabbit/turtle together. Lots to check before condemning motor
     
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  4. slwbid

    slwbid Well-Known Member

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    Thanks all.
    Sr# is 8gro1237 or 8hro1237.
    One is stamped on plate. Other is what cat has to use when looking up parts. Go figure.
    1. I forgot to mention that I had the swivel rebuilt right after I got it.
    2. Yes when traveling, both sides go into turtle or rabbit just fine, but drift to one side is present in both speeds.
    3. Yes I have noticed there is some change in the drift, but not elimination, when I put just a slight amt of pressure on one of the joysticks while traveling. I seem to recall it it is the "Stick IN" function. In fact it almost seem the machine starts traveling a bit faster.
    4. Yes the floor is clean around the travel controls
    5. The weak motor is bad enough that when trying to turn with just that track, lots of time, it will just not go. Does not bog engine at all. Just stops moving unless I help it with bucket or engage other side in opposite direction. The other side will spin the track on the ground, but rarely if ever does the weak track spin while on ground.
    6. Sadly, I dont know how to check the pressures.
    7. I know of only 1 hydraulic pump on this excavator. It is obvious it is not one that came in it/has been replaced.
    8. I recently changed the gear oil. Other than being pretty black, there seemed to be no issues. Pretty clean looking. No trash. No milk.
    9. One other thing worth mentioning: Due to my own ignorance (until reading some related posts recently), I am running cheap hydraulic oil ...whatever is cheapest is what I buy. I think it is probably 10 weight.
    Yes, I can hear yall hissing and calling me cheapskate now - and I deserve it. But I am wondering if I drained the hy fluid and went back with heavier, hi grade hy oil, if the problem might improve.
    If so, can someone make recommendation for this specific excavator...cat 311b, 2002, gray market machine?
    10. I'm no mechanic, but t by e fact that the condition has not worsened in 2000 hours would seem to me that its likely not due to worn travel motor.
    11. I have used it so much and learned to compensate for it, I had about forgotten about it. But a man thinking of buying it, used it for several hours and he noticed it distinctly...and it killed the sale.
    Thanks for any more tips you all can offer.
     
  5. heymccall

    heymccall Senior Member

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    There are pressure tests that need done to measure pump pressures. And a case drain test for the final drive motors. Perhaps someone else can steer you to the testing.
     
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  6. funwithfuel

    funwithfuel Senior Member

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    I haven't seen anyone mention track sag. Are your tracks tight? If you lift one side off the ground does your track hang down below the rollers? The fact that it "speeds up" when you arm in means one pump is not stroking until you pull the added function, that strokes the pump up giving you your extra volume.
    Let's do a simple test, adjust your tracks to spec(see ops manual) raise one side up paint a pad a bright color you can't miss. Travel that track at high speed full throttle. When the colored pad comes around, start your stopwatch. Count three revolutions of the colored pad, stop time on 3rd . Do this fwd and reverse on each side. No other functions just single travel. This will help determine what shape your pumps are in displacement wise.
    BTW do you have a straight line travel pedal?
    Good luck
     
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  7. TVA

    TVA Senior Member

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    Before you do any drastic move check the strainer inside inlet fitting of corresponding travel valve/pedal, or check your pilot pressure on that one.
     
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  8. slwbid

    slwbid Well-Known Member

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    I sure wish I knew how to test pressures.
    I dont if I have straight line travel pedals or not. Nit sure what that means?

    As to checking the following.anyone have a manual that could hel me locate, the strainer inside inlet fitting of corresponding travel valve/pedal, or check your pilot pressure on that one.
     
  9. heymccall

    heymccall Senior Member

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    IIRC, the pump output pressures can be diplayed in the monitor's service mode.
     
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  10. slwbid

    slwbid Well-Known Member

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    Heymccall
    Thanks. I'll try to check to see if that part my monitor still works
     
  11. Champwise

    Champwise Member

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    Travel motors have two big travel pressure lines, a park brake release line and a travel speed line that all go through the swivel to each travel motor. The travel speed line changes the swash angle of the travel motors selcting either high or low speed. Travel motors are supposed to shift to low speed internally even when rabbit mode is selected when travel pressure increases and more torque is required (eg. Tracking up a hill). High speed = low torque, low speed = high torque. An easy way to test if your travel motor is internally stuck in high speed is to boom jack one side at a time, put a paint mark on your track and measure the time it takes to do 3 full revolutions in turtle and rabbit mode on both sides. If one side is always fast regardless of travel speed selected, it's stuck in high speed. This will also explain the drift while tracking. Could just need a valve clean out or replace at best, motor replacement at worst.
     
    Last edited: Oct 6, 2019
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  12. slwbid

    slwbid Well-Known Member

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    I checked track speeds while boom jacked and WOW.
    Here are the results. I'm hoping someone can tell me there is an easy fix.
    Throttle wide open.
    3 Revolutions in turtle:
    Weak side took 25 secs
    Strong side, 32 secs
    3 Revs in rabbit:
    Weak side, 17 secs
    Strong side, 21 secs
     
  13. slwbid

    slwbid Well-Known Member

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  14. slwbid

    slwbid Well-Known Member

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    Champwise you were spot on to suggest boomjacking the machine and no load testing speed of tracks.
    speeds of tracks.
    They are way out of sync in that regard, with the much faster track being the weakest. But they both spoed up when shift to rabbit.
    Can you or anyone tell me how to adjust?
    Thanks so much
     
  15. slwbid

    slwbid Well-Known Member

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    Hey FunWithFuel,
    You were right. The track speeds are way out of whack with each other, with the fast track being the weak one. However they both switch back and forth between turtle and rabbit just fine.
    Thanks to everyone for your help
    Can you tell me how to adjus or something resolve this issue
     
  16. Champwise

    Champwise Member

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    The weak side is faster but there is still a difference between rabbit and turtle.
    Strange how the weak side is within service limit but strong side isn't.
    Try it again in forward and reverse, rabbit and turtle on both sides. Just make sure hydraulic oil is up to temp. track test 311B.PNG
    If that shows nothing you may just have to bite the bullet and test pressures.
     
  17. funwithfuel

    funwithfuel Senior Member

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    So the next thing to do I suspect would be to measure case screen coming off of the motors while free running them off the ground. You have to very large stiff hoses going to the motor those will be your a and B directions forward and reverse. Then you have a slightly smaller hose that is your case drain that is the hose you will need to disconnect and cap and and you will have to put a small hose on the end of that fitting. Keep in mind gravity and take pressure are going to be pushing down on that it would be advisable to have a cap and plug ready when you do this it will leak quite a bit of oil quite quickly. Place the loose end of the hose that you just put on that fitting into a bucket. raise that track off the ground and run that track at speed.
    you wanna see how much oil you collect in the bucket after 3 track revolutions. Ideally what you would want to see is a small trickle of oil filling up the bucket very very slowly and in should be steady no surges. If you get something shooting out like a machine gun or a very high flow that would indicate a worn out motor or a worn out barrel assembly either way you will be looking at a track motor replacements. If your case train is slow steady and even then we have to suspect that you may have a problem in your turning joint. Everything I'm telling you is simple field expedient test that you can do without much equipment. Of course we won't blatantly Condemn anything until we get pressure readings but these tests are basics to narrow down your field of view

    Good luck
     
    Last edited: Oct 8, 2019
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  18. latiff"

    latiff" Member

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    Does this not have to do with the adjustment screws on the both side of the pumps control head.in my experience when one side of it is adjusted more than the other, there's a pressure differentia which allows for drift during travel ,one pump powers one travel motor,except in in implement mode.the pressure from both axial pumps should be gauged to get a clearer view of the situation. My opinion though
     
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  19. funwithfuel

    funwithfuel Senior Member

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    Agreed, but as OP pointed out, gauges were not available at the time.
     
  20. latiff"

    latiff" Member

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    I don't know about him, but if we're in his position I would slack the nut on the adjustment screw (of the slower motors axial pump) on the pumps control head. Insert the Allen key and give it a clockwise turn (90° at a time) ,and check how much speed it gains after every 90° turn . The machine should be boom jacked during this operation. Check the number of threads on the screw so it can be returned to normal should the operation be unsuccessful. Am guessing you don't have a flow meter.
     
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