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Living with M-551 Sheridans, The Suspension, Wheels and Tracks.
   (Ver 2)

 

I first became acquainted with the M-551 Sheridan AR/AAV (Armored Reconnaissance/Airborne Assault Vehicle) at Sheridan add-on training at Fort Knox, Kentucky, in early 1975. I understand that the Sheridan had been in service since 1968 and was originally supposed to replace the M-41 ‘Walker Bulldog’ light tank. Frankly, I’ve always felt that the M-41 is one of the best looking tanks that I have ever seen, but I don’t know anything about what it was like to operate. The M-41 did have a 76mm main gun as opposed to the Sheridan’s 152mm gun/missile launcher, had a gasoline-fueled engine as opposed to the Sheridan’s diesel and was about 7 tons heavier and a few feet taller than the Sheridan. On the other hand, the M-41’s gun was high velocity; the engine put out quite a bit of power and the hull was steel. The Sheridan’s hull was aluminum but its turret was steel.

The Sheridan was definitely a completely different animal from the M-60A1. It was obviously a lot smaller, lighter, faster, more complicated and also amphibious with limitations. Please keep in mind that this article is based on things that I am trying to remember from the mid 1970s. I didn’t have much to do with Sheridans after I was transferred from Fort Hood, Texas, in 1977. If you would like to look at some pictures of Sheridans I would suggest these websites:

http://www.fas.org/man/dod-101/sys/land/m551a1.htm or
http://www.soft.net.uk/entrinet/armour11.htm

The Sheridan’s suspension was pretty simple and easy to maintain. There were five pairs of road wheels on each side, and I think each set had a shock absorber. The idler wheel is at the front and drive sprocket at the rear. The suspension is of the ‘Christie’ type so there are no return rollers. The track blocks were manganese steel with two runner pads bonded into each. The blocks were connected by single pins, hexagonal in cross section and necked down and threaded on each end. The track was ‘live’ which means that the hexagonal sections of the track pins indexed into rubber (?) bushings bonded into the track pads, which had a hexagonal metal sleeve in the center to make contact with the pin. This system is supposed to provide increased track life over ‘dead track’ systems. The only possible drawback is the minimal amount of extra effort matching up the bushings in the two track blocks to get the connecting pin through.

The sprocket had a single set of teeth in the center, which contacted a rectangular hole in the center of the track blocks. Because of this there were actually two center guides cast into each track block, one on each side of the hole. The purpose of the center guides is to try to keep the track centered under the road wheels in order to avoid ‘throwing’ the tracks or damaging the rubber tired road wheels if they happen to get up on top of the center guides. This might occur due to high speed maneuvering on rough terrain or because of a broken torsion bar.

Suspension was by torsion bars, as mentioned, and shock absorbers. A broken torsion bar could be detected by trying to lift a road wheel set with a ‘tanker bar’, which is a steel bar about 5 feet long with a chisel point on one end. A bad shock absorber was harder to detect because the only way I was taught to do it was check the temperature of each shock absorber after some cross country travel. If one or more were noticeably cooler than the others, it/they might be bad. Road wheel hubs should also be checked for unusual heat at every stop in the field or on a road march. Remember to always do this with the back of your fingers. Some people have a tendency to close their hands when they feel heat or electric shocks on the insides of their fingers. This is obviously not a good thing to do.

A dead track block is a track block that has had the rubber bushing connections damaged or worn out. The angle between two track blocks is noticeably different from the others if the bushing is loose or damaged. This condition is actually pretty easy to detect.

The idler wheels at the front of the vehicle are used to adjust the track tension. Track tension is actually very important and I’ve noticed that most of the pictures I have seen of Sheridans have shown them with the tracks too loose. I can’t remember the distance in inches but I think it was about 2 ½. This is the distance that should be between the top of the middle road wheel’s rubber tire and the bottom of the track directly above it. The first three fingers of my right hand are very close to 2 ½ inches wide so I could easily check track tension in just a few seconds, and I did it frequently on all three vehicles in my section. I don’t remember any of them ever throwing a track. If you keep the tracks too tight you can cause premature failure of either the idler wheel hub bearings or the sprocket bearings. Idler wheel hub bearings aren’t really a big deal, but sprocket bearings are.

The track tension is adjusted using a standard grease gun. There is an arm that pushes the idler wheel out against the track and that has three significant features. One is a standard ‘zerk’ (grease) fitting. One is a small bolt. The last is a red line inscribed around the circumference of the extensible part of the arm. The red line should not ordinarily be visible. If it is it means that the track bushings have become so worn that the tensioning arm is at the end of its travel and at least one track block needs to be removed or you will not be able to tension the track properly. The grease fitting is used to increase the track tension and the small bolt is used to reduce it.

A little more on the small bolt. If you need to ‘break’ the track to remove a track block or work on the suspension, remove the bolt and reduce the track tension as much as possible in order to make the operation as easy as possible. This may seem straight forward, but often it is not. The bolt is on the top of the cylinder that is filled with grease but the opening is usually plugged with old grease that has hardened to the point where the idler arm will not collapse by itself. Have a metal can handy to catch the grease anyway or it will make a mess. Don’t use a Styrofoam cup. I’ve seen a number of cases where the expelled grease blew the bottom right out of them. If grease doesn’t start oozing out as soon as the bolt is removed, sterner measures are required. Trying to push the idler wheel back manually hasn’t ever worked for me. I would try a sledgehammer first. If that doesn’t work it may be necessary to get another Sheridan to put its idler wheel up against yours and have them SLOWLY push against it. The other vehicle had better have a driver that you would trust with your life. If you are slick you take this opportunity to get all the crud out of the tensioning arm once it is fully retracted. Pump grease into it with the bolt out until it comes out looking new, then give it about another 20 pumps. It can make a mess but I believe it is well worth the effort. Don’t use air-powered grease guns.

Once the idler wheel is retracted you can break the track. On all of the tanks I’ve worked on this is normally done between the idler wheel and the first set of road wheels. The first step is to connect the track jack. A track jack for a Sheridan is a set of internally threaded curved jaws connected by a threaded section with a hexagonal end. The hexagonal end is turned with a wrench in order to open or close the jaws. It is made to span two track blocks. Because the Sheridan’s track is live the track jack has to be used to change the angle between the ends of the joined track blocks so that the hexagonal track pin will slide out. If the angle between the two track blocks is not within a very few degrees of correct, you will not get the track pin out without using cutting equipment or explosives. If the angle is right, and the track pin was lubricated before it was put in, it can sometimes be pushed out by a hand using the ‘pin punch’.

I’m sure the pin punch has a technical name but I don’t know what it would be. It is the same tool used to punch pins on the M-113 armored personnel carrier. It kind of reminds me of a ‘ball starter’ for a muzzle-loading rifle. It is ‘L’ shaped with the point where the two legs of the ‘L’ meet heavily built up. The short side of the ‘L’ extends beyond the joint somewhere around and inch and is about an inch in diameter.

The way this is done is that you start with the track jack loose enough so you know that the angle between the track pads is too small and the track pin nut on the outside edge of the track removed. One crewman tightens the track jack a turn or so and another taps the ‘L’’s short side extension against the end of the track pin which is in a kind of a cup in the edge of the track pad. If the pin doesn’t move the track jack is tightened again. Eventually the track pin should move noticeably. You might fine-tune the angle after this. If you reach the point where the angle between the track pads is obviously too great and the track pin has not moved you start loosening the track and use a bigger hammer to do the tapping. If there isn’t a big enough hammer in the world to get the pin out at any of the available angles, try breaking it at the next pad. Once the track is broken it is fairly easy to break it on the other side of the joint that won’t come apart and discard everything in between.

Putting the track back together is usually a lot easier. Get the two ends of the track near each other somehow, usually with a rope. Connect the track jack. Tighten until the freshly greased track pin slides smoothly through the track pad bushings and tighten the nuts on the ends. In this case you will probably want to have the nut already installed on the outside end of the track pin. Try to tighten the two nuts so they have the same number of threads showing on each side. A little ‘anti-seize’ on each of the threaded parts probably wouldn’t hurt because the nuts are of the ‘deformed thread’ locking type. This means that the un-flanged end of the nut has been crushed slightly in order to provide a locking effect. These nuts are NOT reusable because they lose more than 50% of their locking ability on the second use.

Once the track is back together just pump grease into the idler wheel tensioning arm until you have the right gap above the center road wheel. You will probably have to add more the first time you take the Sheridan on the road because it seems to pack down a little. If you are conservative or you had to re-use the track pin nuts, use a ball peen hammer to deform the end of the pin that protrudes past the nut. This makes them a bitch to remove but they won’t remove themselves very often. This also works very well with the wedge bolts used in M-60 tracks.

One item that needs to be watched is the road wheel hub lubrication. When I got to Fort Hood we had the type of hubs that were lubricated with OE-30 (Oil Engine 30 weight) and had a sight glass in the center of the hub to show the level or if the oil had become contaminated with water, which would make it look milky. The M-113 used the same system at the time, I believe. One or both of these vehicle types were eventually converted over to lubrication with GAA (Grease Automotive and Artillery). This involved replacement of the hub cover with one that had a zerk fitting, mentioned above, and a pressure relief valve. The seals were probably different as well.

With the OE-30 hubs the oil level was supposed to be half way up the sight glass. The GAA lubricated hubs just required that we pump grease into the zerk fitting with a grease gun until clean grease came out of the pressure relief fitting. You still needed to keep an eye on things because if you saw milky oil or got milky grease out of a hub very often you probably had a bad seal. Hub bearings take a beating and it is best to keep them protected from nasty stuff as well as possible. There are very few lubricants that are as effective when mixed with water as when not. The water problem didn’t always occur because of fording rivers. Remember that we cleaned the vehicles with high-pressure water hoses.

The only other part I can think of is the rear sprocket. I think that the rear sprocket and the idler wheel were both lubricated with GAA. The interesting part about the rear sprocket is that it was directly connected to the drive shafts from the transmission, which had to be removed before the engine could be removed. This was actually very easy. There was an aluminum plate that covered the center of the rear sprocket hub. I think six bolts held it on. Remember that this was direct drive and not a geared final drive like an M-60. The axle shafts looked like very large nails with no points. There was a splined section on the small end and the head was also splined. To remove them we just removed the cover plate and pushed them out from inside the hull, making sure there was someone there to catch them. There were times that they might be in a bind and I think they had a threaded hole for an eyebolt, but I don’t remember ever having to use one.

Putting them back is was a different story. I think that the inner splines that went into the transmission were quite a bit longer than the outer splines. I’m not sure that we could turn the part of the transmission that the splines went into. I don’t remember ever having as much trouble getting the drive shafts back into a Sheridan as I had trying to get M-60 final drives mated up but I guess, if worse came to worse, you could break track on both sides and lay it out flat, push and twist the axle shafts in until they mated up with the transmission, then rotate the sprockets until their inner splines mated up with the drive shafts outer splines, push them on in and put the cover plates back on. I don’t ever remember having to do that, but I didn’t ever have an engine or transmission replaced either. I had them out a number of times, but they were never replaced.

I almost forgot roadwheel serviceability. Roadwheels are supposed to be rated as 'unserviceable' if there are any places where the molded on rubber is missing for the full width of the roadwheel. Beyond that, if the molded on rubber is missing for half or more of the width of the roadwheel over some specified arc, or is obviously severely damaged from center guides poking into it, or even oxidation due to ultra violet light, or has any elongated bolt holes, you should probably get it replaced.

NB: Much of what Rory says above is equally applicable to the M113 series of vehicles.

My thanks yet again to Rory.

 

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