Brake drums are the end of the mechanical brake chain. Drums dissipate the heat developed to overcome the speed and weight of the vehicle. Drums wear. A very popular size drum on tractors and trailers in the class 8 vehicle is 16.5 inches inside diameter, 7 inches in depth. These drums are generally made of cast iron. There are also steel drums with a cast inlay. Both of these wear as used. A simple method to inspect for check wear is to simply put your finger into the drum and feel if there is a ridge this will tell you if the drums are down beyond the MFG’s specs, if the ridge is 1/16th of an inch (the thickness of a quarter) it is worn down to 125/1000ths of an inch, that is the spec limit recommended by the MFG. The thinner a drum is, the easier it expands. Thinned drums have more propensity to crack, and can indeed come apart. It’s a common practice to use drums through the life of several re-linings. It’s an economical practice, however needs to be monitored closely. A cracked drum loses brake potential and can cause a catastrophe. Drums, to be mated with new linings need to be carefully inspected as they can be worn in a number of ways: deeply grooved, heat checked, egg shaped, slanted (more wear at the bottom), worn beyond the MFG’s specs –all of which can and will affect braking.
Knowledge of how to inspect is vital, for safety’s sake. Inspection after an accident, properly done, can be a critical factor in the case and often is. It is indeed a rare driver that really inspects the equipment assigned to him or her. The priority is to get going as the drivers get paid by the mile and generally do not want to get dirty. Therefore drivers can’t answer questions on the brake system. Simple questions like, “How many clevis pins in your brake system?” “Is adjustment on the left side of axle #3 the same as on the right?" "How many drums?" "How many shoes?" "How many brake blocks?" "Are all the drums ok?" "No cracks?"
There are 20 shoes on a 5-axle semi. Each shoe has 2 blocks (linings). This means 4 blocks per wheel, 8 blocks per axle, for 4 axles. The front axle can have only 2 linings per wheel. Brakes are quite often replaced on one axle at a time. This is acceptable, however the driver need to be informed and have the knowledge that something has changed in the vehicle’s braking system and that the equipment has an unbalanced brake potential. Therefore, the driver has to pay more attention, especially if old drums are used. It takes some time (miles) for the new linings to match to old drums.
It is a rare company indeed that truly educates their drivers on proper inspection or proper use of the air brake system. It is also a rare for a manufacturer to put out educational material for drivers. It is common for them to deliver material to mechanics. In the driver’s world it is a wrongful assumption that because a driver has a CDL, they have the necessary knowledge to operate equipment safely. Brake componentry is usually a very dim part of drivers’ and safety departments’ knowledge, and therefore very productive area for knowledge questions.