Truck Topics

The Tool Box - Axle & Wheel Seal inspections

The drive axle is the rear axle that has a drive shaft attached. Some trucks have two drive axles; this is called a twin screw tandem. These axles are basically the same as a single drive axle, with the exception, there is a drive shaft between the axles. The steering axle is the front most axle and does the steering of the truck, hence the name. This is what we need to watch for on axles, twin screw or single drive, trailer, lift or steer.

Pinion seals – the pinion is the shaft on the front of the axle (and on the rear of the front axle of a tandem) that the driveshaft is attached (with a yoke and u-joint). The pinion seal is around the rear most part of the yoke, installed in the front of the axle housing. These seals (one per yoke) can leak, and if they leak long enough can cause a bearing failure and a major expense to repair. To check for leaks just follow the drive shaft to the rear axle and around the yoke for fresh oil. If there is a little fresh oil, it may wait till you can get home. If there is a lot, find a shop. The drive axles are the only axles with pinion seals. In either case you should check the oil level and refill with the proper oil. I will explain checking and filling shortly.

Wheel Seals – We have all heard this term but do you know where they are?  Wheels don’t really have seals but the hub that rotates with the wheels attached does. This seal is very hard to see as it is hidden by the brake shoes. To see if it is leaking you may need a flashlight to shine into the brake drum and around the brake shoes. Here again, look for fresh oil. This one if it is leaking, find a shop. Sometimes it’s very obvious if a wheel seal is leaking as the brake shoes are all oily and the oil is slinging out onto the inner sidewall of the tires. In a case like this the brake shoes must be replaced also. Failure to replace a leaking wheel seal is a D.O.T. violation. This could cause a bearing failure on the wheel end of the axle and you don’t want to have to find out what this will cost to fix. 

As before, any kind of seal leak means we’ve lost lube oil. On most drive axles the check plug is a square protruding about half way up from the bottom on the back of the axle usually off to the right looking at it from the back. Take a crescent wrench, adjust to fit and remove plug; it will be a ½” pipe plug, sometimes with a magnet attached. The oil should be right at the bottom of the plug hole, or just coming out. To add oil, fill thru the opening till the oil comes out, install plug and tighten.

By the way, the steer, trailer and lift axles have wheel seals also; check them the same way, but fill them from the caps with plug on outside  (under the chrome hub caps). There is a window in the face of the cap with a rubber plug that removes easily. Use caution as this window can break, and fill to the line on the window thru the plug hole.

Checking the axles needs to include the suspension for loose parts/bolts.  These items are noticeable being loose because the loose parts leave rust tracks around them. You may not be able to move them by hand but you can tap them with a hammer; if they move, find a shop. Loose parts on the suspension are a D.O.T. violation and it’s much cheaper to get this done in a shop than at the scales. Check all the bolts, especially the u-bolts. These are the u-shaped bolts (hence the name) that go over the leaf spring on the front and rear of the axle (the nuts are on the bottom). Having the u-bolts loose is not only dangerous but costs you fuel, as your truck can become misaligned, steer, trailer and lift axles as well.

Check the brake shoes – these are a curved piece of steel with a friction material riveted to it that rubs against the brake drum to slow you down. The friction material must be thicker than ¼”, with new shoes being almost ¾” thick. If the shoe gets any thinner than ¼” the rivets will rub on the brake drum and ruin it, also taking away the ability of the shoes to do their job. Another D.O.T. violation if shoes are worn out. The steer, drive, trailer, and lift axle brakes need to be checked during inspection.

Look at all those wires and air lines that are attached to the axle or any part bolted to the axle for “chaffing”. This is a term used by D.O.T. when they find a worn spot in a hose or wiring harness. If you find it first, it’s a good idea to replace the airline and repair the wire properly then, get some cable ties and tie the parts so it won’t happen again and save yourself some D.O.T. hassles.  This applies to all axles-drive, steer, trailer and lift axle.

A good visual inspection done regularly will prevent a small problem from becoming a very lengthy expensive one.

Until next time keep an eye on things.