BRAKE DRUM INSPECTION.

No Brake Inspection Is Complete Without Removing The Drums The typical drum brake assembly consists of the drum itself, a wheel cylinder, a pair of shoes, return springs and hold down springs for the shoes, a backing plate and one or two anchor pins to support the shoes, a self-adjuster mechanism to maintain lining clearances, and a parking brake mechanism. The drum is usually made of cast iron but some are aluminum with a cast iron liner. The drum provides the friction surface against which the linings rub when the shoes are pushed outward by hydraulic pressure in the wheel cylinder. As the linings rub against the drum, they generate friction to slow the wheel and bring the vehicle to a halt. This generates a lot of heat, which the drum absorbs and dissipates so the linings don't get too hot and begin to fade. Some drums have cooling fins that increase their cooling capacity. The drum also shields the internal brake components from road splash and dirt. There may also be a spring wrapped around the outside of the drum to help dampen vibrations and noise. DRUM REMOVAL The drums should be removed anytime you suspect a brake problem or need to inspect or service the brakes. You should still remove one or both drums and take a peek inside. Why? Because you can't tell a thing about the brakes without looking inside the drums. The drums, linings and hardware must all be in good condition to provide safe braking. So don't take chances. Always inspect all brakes. The first step in removing any drum is to back off the shoes so the linings will clear the drum. This is necessary because as the drums wear, they often develop a ridge that will hang up on the shoes when you attempt to pull the drum off. On most drum brakes that have a star adjuster mechanism, access to the star wheel adjuster is usually possible through an small slit or hole in the backing plate. On most applications, the adjuster can be turned to loosen the brakes by working the end of a screwdriver or brake spoon tool against the star wheel. To loosen the brakes on some applications, you may be necessary to insert a second screwdriver through the opening to hold the lever away from the star wheel so it can be turned in the opposite direction. Once the shoes have been backed off, you can remove the front drums on older rear-wheel-drive cars and trucks by (1) pulling or prying the dust cap off the hub, (2) removing the cotter pin and lock ring, and (3) backing off the spindle nut that holds the outer wheel bearings in place. The drum will then slide right off the spindle. If a drum is stuck and won't come off (and isn't hanging up on the shoes because you forgot to back off the adjuster), use care because cast iron is brittle and can be easily cracked if you try to beat the drum loose with a steel hammer. To remove a stubborn drum, use a drum puller and/or an air hammer with a blunt tip against the front of the drum to vibrate it loose. Or, use a large hammer to carefully rap on the front (never the rim flange) of the drum to help dislodge it. CAUTION: Drums will sometimes pop free with considerable force when removed with a puller. Also, don't let the drum and puller fall onto the shop floor because the resulting impact may damage the drum you've worked so hard to remove. CHECK FOR WEAR Once the drums are off, check them carefully for wear. Stamped or cast on the face or outside of most drums is a "maximum diameter" or "machine to" specification, which refers to the maximum allowable inside diameter of the drum that is permitted after the drum has been resurfaced. The drum may also have a "discard" specification. If the drum is already worn to discard spec, or can't be remachined without exceeding the maximum diameter limit, discard it. Measuring the inside diameter of a drum requires a special drum gauge. If you don't have one, get one. If the drum's inside diameter is at or beyond the maximum specification, or cannot be resurfaced without exceeding the maximum specification, it must be replaced. Most passenger car and light truck drums are designed with enough metal to allow about .060 inch of wear (or resurfacing) before replacement is necessary. Why replace worn drums? Here's five good reasons: 1. Liability. Thin drums are dangerous drums. If not replaced, the drum can wear to the point where it may literally disintegrate. And if the drum fails, so do your customer's brakes! 2. Reduced braking efficiency. The increased inside diameter of a badly worn drum won't match up with a new set of linings unless the linings are arced (and who does that these days?). Consequently, the linings only make contact in the middle which reduces their effective braking ability. 3. Increased pedal travel. Wear increases the distance between the shoes and drum. If the drum is worn excessively, the self-adjuster mechanisms may not be able to take up all the slack resulting in a low pedal. There's also the chance that the wheel cylinder may overextend to the point where it leaks or comes apart, causing brake failure. 4. Brake fade. Thin drums can't absorb and dissipate as much heat as thick ones. There's less mass in a worn drum so the brakes may overheat and fade much sooner. 5. Noise. Thin drums can contribute to noise by failing to dampen vibrations. DRUM INSPECTION Drums also need to be inspected for the following conditions: Minor pitting and scoring are acceptable as long as the grooves are not too deep and can be removed by resurfacing. Grease or oil contamination must be removed before the drum is reused. Cracks of any kind are dangerous and mean the drum needs to be replaced. Cracks can result from thermal stress or mechanical stress. Either way, they weaken the metal. Out-of-round or an egg-shaped drum can result from applying the parking brake when the drum is hot. The locked position of the shoes prevents the drum from contracting normally as it cools resulting in permanent distortion. This can cause a pedal pulsation when the brakes are applied. Drum runout should not exceed .005 in. If the run-out cannot be removed by resurfacing, replace the drum. A "bellmouth" condition can occur in wide drums if the inner edge of the drum distorts outward. The result is uneven shoe-to-drum contact, uneven lining wear and reduced braking efficiency. To check for this condition, measure the inside diameter of the drum close to the inside and outside edges of the drum. If there's a difference of more than .005 in., the drum needs to be resurfaced or replaced. Barrel wear on the drum where the center portion is worn more than the edges. If the distortion can't be removed by resurfacing, it will have to be replaced. Hard spots or glazed spots in the drum. These are caused by excessive heat that brings about metallurgical changes in the metal. Hard spots can be identified by raised or discolored patches on the drum friction surface. Hard spots can cause chatter, pedal pulsation and grabbing when the brakes are applied. Resurfacing will shave the tops off any hard spots, but they usually come back once the drum is returned to service and starts to wear again. The only permanent cure for hard spots is a new drum. DRUM RESURFACING If the drums are rough, they need to be resurfaced to restore the friction surface to like-new condition. Rough drums will eat up a new set of shoes very quickly, so don't skip this important step. Drums should always be resurfaced in pairs. Never do one drum and not the other because differences in diameter can affect side-to-side brake balance. The inside diameters of both drums should be within .010 inch of each other. A greater difference may cause undesirable variations in brake force side-to-side. If you're installing new drums, most come from the factory in a finished condition and are ready to install. Resurfacing a brand new drum is unnecessary and needlessly shortens the drum's life. Even so, some technicians will still take a light cut on a new drum just to make sure the drum is true (concentric). Experience has taught them that new drums aren't always as round as they're supposed to be. When drums are resurfaced on a lathe, make sure they're mounted properly and securely to minimize runout. A silencer band should be wrapped around the outside of the drum to dampen vibrations and reduce the possibility of creating chatter marks as the drum is resurfaced. Then remove the least amount of metal that's necessary to restore the drum surface, and no more. This will prolong the life of the drums. As a rule, the best drum finish is achieved with a slow, shallow cut. The smoother the finish, the better. A lathe speed of 100 to 200 rpm with a cross feed rate of no more than .002 inches per minute and a depth of cut of less than .002 inches will usually produce an ideal surface. If the drums are turned too quickly, the tool bit can leave tiny grooves in the surface which can chew up a new set of linings as well as make noise. One way to tell if the surface finish is in the recommended range of 80 microinches or less is to try writing your name on the friction surface with a ball point pen. If you see a continuous line of ink, the finish should be smooth enough. But if the ink line is broken up into little dots, the surface is too rough. DRUM REINSTALLATION A common mistake that is often made is not cleaning the drums after they've been turned before they go back on the vehicle. Resurfacing leaves a lot of debris as well as torn and folded metal on the inside surface of the drums. Much of this debris will be knocked off by the brake linings when the brakes are first applied, but some of it will become embedded in the linings and may contribute to a noise problem. So for best results, scrub the inside of the drums with soapy water and a stiff brush. And if you're really picky, lightly sand the inside of the drum with #120 or #150 grit sandpaper before you wash them. Once the drums have been resurfaced and cleaned, keep your greasy fingers off of the inside surface. Handle the drums with care and do not store them on end or drop them, otherwise you may knock your drums out of round. If the drums are the type that have integral wheel bearings, inspect the bearings and races carefully. Replace the bearings if they're not in perfect condition. Clean and repack the bearings with a high temperature wheel bearing grease. Don't be tempted to reuse the old grease seals. They'll probably leak. Throw the old grease seals away and install new ones, making sure you drive them in squarely without damaging them (use a seal driver). Also, check the condition of the wheel studs, and replace any that are broken or damaged. If you're replacing the old drums with new ones, use drums from a quality supplier. Some cheap offshore drums are made from inferior grades of cast iron which do not wear well and have hard spots. There may also be problems with fit and finish. Too rough a surface finish inside the drums can be overly aggressive and upset brake balance, increasing the likelihood of rear wheel lockup and skidding under hard braking. When you're ready to reinstall the drums, you can use a "drum/shoe gauge" to preadjust the shoes to the inside diameter of the drums. If the shoes are out too far, the drum will jam as you try to push it over the shoes. Never try to force a drum into place because doing so may damage the linings. Pull the drum off, turn the adjuster to retract the shoes and try again. Once the drums are in place, the shoes should be adjusted until they just clear the drum (little or no drag felt when the drum is rotated by hand). If you forget to adjust the shoes, you may end up with too much pedal travel and no brakes! Reducing the clearance between the shoes and drum is necessary to minimize the distance the brake pedal has to travel to apply the brakes. FORKLIFT OPERATION AND SERVICE. BE SURE TO VISIT THE HOME PAGE FOR INFORMATION ON LIFT TRUCKS REPAIR, SERVICE, TROUBLESHOOTING, AND EVERYTHING YOU NEED TO KNOW ABOUT FORKLIFTS AND THEIR USE.