truck parts :Turning Brake Drums

The following information on major components of the air brake system will help you know what to look for during
your brake inspection and service.
 Slack Adjusters: Slack adjuster should be checked for any play, looseness indicates a worn worm gear. A worn worm gear will allow movement of the adjuster without any movement of the S-cam. The clevis bushing in the adjuster should also be replaced with each brake reline. Always make sure that slack adjuster length is the same on each side of an axle.
 Brake Chambers: Each of the service chambers must be of the same size across the vehicle's axles and within each axle set. The push rod actuation should be straight out of the chamber and the rods should be of equal length. Service chamber springs can break or weaken and will cause the pushrod to fail to retract upon release. If that happens, the brakes will not be completelyreleased and the brake linings will end up dragging against the drum. The yokes also should be checked for wear and replaced if worn at all.
Brake Drums: Look for drums that are scored, bell mouthed, out-of round, severely heat checked or in generally poor shape. Any of those conditions can lead to a broken drum and brake failure. A number of fatalities and injuries have been attributed to broken brake drums because the components were left on the vehicle long after they should have been replaced. Poor drum condition also will cause brake linings to wear rapidly. Because worn drums have less material to act as a heat sink, higher temperatures will occur whenever the brakes are applied and will also shorten the brake lining life. Installing replacement brake shoes on a vehicle equipped with badly worn drums can shift the initial lining contact from the center of the shoe to the camshaft
end, reducing lining to drum contact and increasing lining wear. Most OEMs do not recommend turning or brake drums, but drums can be turned to clean up a drum's surface if the diameter is within specifications. If you will be installing resurfaced drums on a vehicle, make sure to check for the possibility of cam rollover or a sticking cam due to the increased diameter of the drum. Remember, you have ?" lift. Cutting a drum to .100 over will use nearly ?' of that lift.
Brake Linings: Quality brake linings replaced in axle sets is the best assurance of balanced and reliable brake operation. Combination friction linings are used, make sure that the leading and trailing shoes are properly located. On most common installations, the forward or primary lining must be installed on the forward shoe. Remember that following the rotation of the drum, the first shoe after passing the cam is the forward shoe.
 Return Springs: Brake shoe release depends on the return springs being strong enough to pull the brake shoes back to their "at rest" or normal operating position. Broken or weak return springs will allow the brake shoes to constantly rub on the drum and lead to premature lining wear. When one shoe is seen to have much more wear than another on the same wheel, it is usuallyweak brake springs. At Franklin Truck Parts we often see broken brake springs that have caused extensive damage. A broken spring will allow the roller to come out and tear up the lining, the shoes and even break the drum. Expensive! Replacing the return springs with every brake overhaul is cheap "insurance" and will prolong brake lining and drum life.
 Camshafts: Two types of camshaft heads are common, the "S''camshaft shape and the"T"or flat head camshaft shape. The "T" head camshaft head camshaft usually is found on front axles. With the S-camshaft there will be wear in the bushing area if it's not properly lubricated. Wear in this area will not allow a snug Fit in the bushings and that can increase brake output and reduce brake lining and drumlife. Flat spots on the camshaft head will interrupt the movement of the shoe into the drum. The camshaft head takes the force of the trailing shoe roller. This side of the camshaft head may wear more as a result of the force of the roller.
Wear in the bushing area, flat spots on the head or distorted splines mean the camshaft needs to be replaced. The camshaft head

When inspecting the brake system or performing a brake overhaul, it is important to check condition of all parts.
Those showing even minor wear should be replaced. This insures that the system will function as smoothly and
safely as it was originally designed to.

Air brakes on trucks are a system made up of of many separate parts, all of which must be in good condition to
provide reliable operation. More because of Safety violations of the air brake system are the number one reason
for trucks being pulled out of service.

DO A COMPLETE BRAKE JOB

Camshaft operated brakes are very different. There is no fixed adjustment, and The anchors are not adjustable to allow for excessive diameter in the drum. The ?" geometry in standard "S-Cam" brakes (4515 or 4311) calls for replacement of the lining when worn to ?" thickness. The camshaft has a ?" lift factor (?" minus ?" equals ?"-the Cam Lift Factor) and is installed to the various manufacturers' geometry setup. If a drum is turned out to 0.060" oversize and standard lining is installed, loss of initial lining to drum contact is sufficient to cause premature brake fade or a driver complaint of a hard to stop vehicle.This can be eliminated by installing oversize block, so that necessary square inches of contact to the drum is restored by replacing the drum loss with lining gain. However, this will still cause some premature lining fade due to the "mass" change in the drum. The thinner the cast iron on the contact surface, the better chance of fade-and the greater chance for breakage. Due to the "fixed" geometry, a cam brake can only utilize up to 0.5" lining thickness-die cam only lifts 0.5" and the anchors are not adjustable-leaving usable lining when "cam rollover" occurs. Therefore, if a drum is 0.060" oversize, 0.030" of usable lining will remain on each shoe. Since oversize lining cost is 10% to 30% more than standard lining, Cost Per Mile is a key issue in lining to drum contact issues. .Is it worth it? Many over-the-road fleets do not turn drums, but wear out two sets of lining to one drum and then replace the drum. Some fleets that like to turn drums for concentricity, only turn them once to approximately .060" to .090." This is determined by the individual fleet's cost analysis and their liability concerns. It is very important to remember that the best brake performance will be when the lining to drum contact is at 90 to 95%. The square inch to BTU Ratio will keep the lining surface at a lower temperature and help eliminate the possibility of brake fade occurring. The heavier the drum mass, the better the heat dissipation to the atmosphere.

Hydraulic brakes systems with their adjustments for shoe positioning can compensate somewhat for oversize drums. The fixed adjustment allows for enlarged diameters: Simply adjust the star wheel out to compensate, and the wheel cylinder will do the rest. This will work only to the maximum turned diameter stamped on the drum.

The issue of turning brake drums to greater diameters and compensating with oversize lining is a confusing one for mechanics who also work on hydraulic systems.

Most manufacturers do not recommend resurfacing brake drums. On the other hand, turning or cutting a drum to remove small surface defects and heat checks may improve performance. It is critical that the drum in kept within the maximum diameter limits.

TURNING DRUMS

Two types of camshaft heads are common, the

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