Monday, December 15, 2003

Check that skid steer loader drive line!

This is a very broad topic of which I can only
begin to touch the surface. I will relate some of my past experiences where I have found problems with the skid steer drive line. Typically, the drive line consists of the drive coupler, splitter box, hydrostatic pumps, drive controls, motors, drive chains, chain boxes, axles, bearings, axle seals, and wheels . This discussion will focus primarily on the modern skid steer loader. There have been many variations of drive systems and I am familiar with a small subset of them. Again, I am only sharing some of my past experiences, I am not an expert on the topic. The engine could be called the most major component of the drive line but there is so much about the engine that I will reserve it for a future discussion.
The drive coupler connects the engine to the drive pumps. The coupler is a shock absorbing flexible shaft or plate. The shock absorbing flexible shaft or plate is where the problem occurs. It must be made of a material that cushions and /or bends, a condition that can eventually deteriorate. Worn parts leave gaps that result in a knocking noise and/ or a vibration that can be present any time the engine is running. Do not be too concerned about a soft chatter because the coupler can last a long time at this lower level. However, if the drive coupler a has a loose or worn universal-joint, it can come apart and wreak havoc in the surrounding area, as well as your checking account.
Some larger machines use a splitter box. The splitter box is located between the drive coupler and the hydrostatic drive pumps. If you are considering a machine with a splitter box, be sure to check the oil level of the splitter box. If the oil level is too high, the oil is coming from somewhere, most likely, a drive pump with leaky seals. It is frustrating to fix because you don’t know which pump is leaking, therefore you must fix both of them. The first thing to look for is an oil leak under the machine. If there is an oil leak then naturally you want to know where it is coming from. Lift the seat up and lock it in the raised position. You will find a dip stick for the splitter box in the center of the splitter box between the pumps. If the oil level is way up over the full mark then you know that the pump seals need to be repaired. Splitter boxes don’t hold much oil so there is always the possibility that somebody just added too much oil one day and never corrected the mistake. It is always a possibility, (yeah, when pigs fly!)
Hydrostatic drive pumps and motors are tough to diagnose, because they may look good, sound good, but be bad. That is why there are auctions. “Caveat emptor”, buyer beware. Sure you can get oil samples, but there is an old saying in the American car engine motor head circle that is “there is no substitute for cubic inches” meaning bigger is better. With hydrostatic pumps and motors bigger is better, the bigger the time you can sit in the seat driving the machine is better. Working it, digging, driving, counter rotating, getting it hot is the best thing you can do. A cold drive system may operate very different than a hot one so you must work it, get the thermostats to open and make the radiators do what they were designed to do. You will get it hot after you own it so you better get it hot before you own it. Find a good pile of anything offering resistance. While under a load, make sure the wheels keep spinning. It is normal for the hydrostatic pumps to have a high pitched whine under a torque load. With a full bucket pushing forward, the wheels should never stop pushing. The machine should drive straight forward and reverse with out a constant correction in the steering. The machine should counter rotate, meaning the left side should spin in the opposite direction from the right side so that the machine should turn in a complete circle within its own length. I will never forget the time I was just about to say “Yes I’ll take this beautiful Case 1845C” it ran great, looked great, had no odd noises, counter rotated normally, drove in a straight line forward and reverse, didn’t creep or have any oil leaks. I took it into a pile of dirt and just as the pile filled the bucket and I started to push, the left side quit, both left wheels stopped moving while the right side was saying “come on lets go, what is the hold up?” The left side with out warning just basically said “that is it, I am done, don’t push me any more”. I was offered the same machine about a month later by a dealer that had traded it in with no idea what he had done. Famous words of Mr. T “I pity the fool”.
Some machines have a two speed built into the hydrostatic drive, if it has a two speed then it should work. Test it. It will kick up speed when in high gear but don’t try to dig in high gear, high speed means low torque. Be aware that some machines, Bobcat for instance, has the two speed switch but does not necessarily have two speed built into the drives. That could prove to be a costly mistake, since the two speed is an expensive option while the switch with the rabbit and turtle on it probably cost about two dollars.
There are many types of drive controls (the handles you use to drive the machine), too many to describe. What is important is that the controls need to be secure, without free play. On a hydrostatic drive machine, free play in the controls can allow the lever to drift into forward or reverse thus letting the machine suddenly drift fully into forward or reverse. Picture that with no operator in the driver seat and a parking brake that doesn’t hold. It’s a runaway skid steer loader! This is not funny, with an uncontrolled two to five tons on the loose! Call an ambulance because you’re going to need one! If the control linkages are getting sloppy, or off center it is a very dangerous situation. Control linkage can be adjusted and tightened but people would rather spend their money on Starbucks coffee. That’s right, about $3.50 to $5.00 per knuckle joint would tighten some of those linkages. One of the first things that I want to know how to do when I start a skid steer loader is, HOW DO I TURN IT OFF!!! I have started many skid steer loaders when the immediate thing that happened as soon as the engine was running, the machine started to move, and not because I wanted it to! I have crashed a skid steer loader and it is not fun. “I wasn’t at fault officer”! I just started it up and it took off, crashing into the skid steer next to it! I X’ed that one off my list and went home to change my underwear. Please be careful folks, this is a very dangerous business.
The drive motors are mounted to the chain boxes. The chain boxes hold the chains, sprockets, usually the parking brake mechanism, lubricating oil and axles. The typical system has one motor per side, the motor spins a shaft with a sprocket mounted on it. The chain runs from the sprocket to the opposing sprocket attached to an axle that goes out of the chain box to the wheel. The maintenance of the chains involves moving an axle or a chain tensioner to keep the chains tight. When the chains have stretched beyond the adjustment limits then it is time for new chains. The sprockets should be replaced if there is a severe wear pattern, badly worn sprockets can prematurely destroy new chains. In the rare instance that I inspect a machine with a chain problem the symptoms are usually very obvious. The side with the bad chain can lock up, jerk, rattle, skip, crunch quite loudly, and/or not move at all. In extreme cases the chain gets bunched up between the sprocket and the chain case, breaking the case open and the guts fall out onto the ground. That could be a clue. I have been able to hear a small amount of chain and sprocket rattle especially on larger machines traveling at a high speed, and I will say that is okay. However, if it is excessively noisy, get in there before it comes apart and gets really expensive. Like the splitter box, I have seen some chain boxes that were over filled with oil. In this case, it is because the drive motor seal failed and the oil from the hydrostatic drive motor filled the chain case. The owner of this machine should have noticed that the hydraulic oil level had dropped but didn’t know where it went, because the chain case can hold about five gallons of oil. “Fire the son of a gun that is siphoning the oil out of my machine!” “Oh sorry Ralph, you can come back to work, we found the lost oil in the chain case, but you lost your profit sharing”. The unsuspecting purchaser of this machine will get a surprise one day because the oil will start spewing out of the machine. The overfilled chain box will leak through a chain case breather hole or a blown axle seal. Two more quick points about chain cases, there is always the possibility that all the oil has leaked out and the wheels have been cleaned from use (catastrophic failure is inevitable). Creamy white oil in the chain box is a sign of water contamination and I would change it. Water typically enters the chain boxes from pressure washing around the access covers
That brings us to the axles, and you thought we were done! A quick check of axle seals is very simple. Typically I haven’t found bad axle bearings without a leaky axle seal. A bad axle bearing is not going to be easily spotted unless the machine is raised so that a person can check the wheel for movement. I would suggest that driving the machine and listening for a rough grinding sound would be a good way to determine a bad axle bearing, however, it is difficult to hear over the hydrostatic drives and the diesel engine. An easier check is to look for a leaky seal. With the machine not running and the parking brake engaged, look at the inside of each tire at the lower side where the tire meets the ground. Is there a trail of oil running down the inside of the tire? If so then the red flag should go up and the inspection moves in for a closer look. The first thing I would look for while looking at the oil soaked tire: is there twine or any other item wrapped around the axle seal? It is very common to have a foreign material get wrapped up in the axle destroying the seal. This would be a good thing because now you have the reason that the seal leaks. If the axle seal is just leaking, with no foreign material wrapped up around the axle, it could be because of the overfilled chain case as discussed earlier or an axle bearing has gone bad. Next, I look for a dipstick for the chain case (if there is one), check the oil level and if it is overfull then there might be a drive motor seal failure. If the oil level is normal or a little low because of the seal leak then replacing the seal may be all that is necessary. You are not out of the woods yet, because replacing the seal may destroy the bearings so with a seal you end up replacing the bearings anyway.
Parking brakes are usually integrated with the drive assemblies. There are many different forms of parking brakes and they are electronically or manually applied. The problem with most of the manual brake assemblies is the operator operates the machine with the brake applied. The electronically applied brake gets destroyed because a sensor or a component like a seat belt switch goes bad and applies the brake while the machine is in motion. I attended an auction recently when Sylvester didn’t know to reattach the seat belt on the John Deere 240 to turn off the parking brake. I could hear the parking brake just grinding away on its self as the operator throttled up more and more just to get over the ramp. I could hear the auctioneer saying “winning bidder internet number 1234 going to West Virginia”. Internet bidders are real gamblers, remember what Mr. T. said earlier. The guy probably got a good buy, until he pays for that John Deere parking brake! Ouch! It is very important to have a working parking brake on a skid steer loader. Do not leave a skid steer on a hill depending on the parking brake, instead, park next to a big tree or a Cat D8 dozer, something immobile. A runaway skid steer is like a loaded gun, somebody or something is going to get hurt. I have sold more than one bucket because a skid steer ran into something and destroyed the bucket, sometimes with nobody in the machine! Are we done yet?
Okay, I’m going to wrap this up with a few more things. Wheels, look for wheels that have been destroyed and straightened, they may not hold air and are foam filled. Foam filled tires are very heavy and cost more to replace than standard air filled tires because they have to be cut off. If you need flat proof tires that add weight to your machine then use the foam fill process. Otherwise I would avoid them. Look at the lug nuts. If they have been loose, then the holes in the wheel could have been elongated and they will never stay tight, figure on buying a wheel, new wheel studs and lug nuts. I recently inspected a JD 250, the left front wheel was literally being held on by about 4 threads of the studs! They were using the machine while I was on the jobsite. I pointed it out to the operator and said “you better tighten this wheel up. It is going to fall off” His reaction was that they tighten them up every day and they just loosen again. Ask me why I drink! The lug nut holes on that machine looked like slots with metal shavings galore. While on the subject of wheels, this is a good time to mention that I always like to get behind the machine and visually align the wheels. They should be approximately aligned. You should be able to take a straight edge and hold it to the outside edge of the rear wheel, then the straight edge should align approximately with the front wheel outside edge. If they are not aligned, it could be as simple as a replaced wheel with a different hub depth or a reversible wheel that is reversed. Many skid steers use reversible wheels, it makes the width of the machine adjustable by a few inches. (Reversible wheels are identified by having a valve stem on both sides and the lug nut holes are tapered on both sides). The worst case scenario is that the axle housing has bent or broken and then the axle is working on a different plane so the wheels are out of alignment. This situation may not be recognizable with the straight edge test but the eyeball test will work. Look at the angle of the wheel, if the bottom of the wheel is squatting out from center or if the top of the wheel is closer to the housing, the chain case is probably bent. More than one time I noticed the oil leaks around the rear of the machine but not from the axle seal, a closer look revealed that the chain case had ripped out at the base, repaired by welding angle iron on the corner and sealing it up with silicone. This is a fairly common repair. On other machines I have seen the front axle housings ripped nearly off the machine. A factory supplied plate that increased the metal thickness in that area gets welded around the complete axle housing. I’ll bet some engineer didn’t enjoy his Christmas bonus that year.
I am sure that I am not done, I will recollect some instance that needs to be related to this driveline report and that is exactly what I will do. Check back and read the whole convoluted thing again maybe I put a comma somewhere or added a little ditty about the time the Bobcat was upside down in the pond, sorry Tom I know that scared the heck out of you.
Checking out a skid steer loader is very dangerous business. Do not do it if you are not familiar with the operation and basic safety procedures. I want to know about other people’s experiences in this field. Feel free to send me an e-mail. Good luck.

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  1. thank you for the information.

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