|Turning Brake Drums
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.
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.
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.
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 ¾" geometry in standard "S-Cam" brakes (4515 or 4311) calls for replacement of the lining when worn to ¼" thickness. The camshaft has a ½" lift factor (¾" minus ¼" equals ½"-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.
DO A COMPLETE BRAKE JOB
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.
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.
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 ½" lift. Cutting a drum to .100 over will use nearly ¼' 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
and the shaft should be inspected for flat spotting, cracks, wear and corrosion. Also, check the camshaft head surface, bearing journals and spline. The camshaft bracket should be closely examined for broken welds, cracking, loose mounting fasteners, bending or misalignment.
Camshaft Bushings: The bushings are designed to hold the camshaft straight and keep the brake shoes centered inside the drum. Worn bushings allow the camshaft head to drop and can cause the bottom brake shoe to drag on the brake drum. Also, push rod travel will be wasted in centering the brake shoes inside the drum before applying equal force. It is recommended that camshaft bushings and seals be replaced each time the brakes are relined to assure a tight, controlled camshaft position and avoid lining contamination. Note that both grease seals should be installed with their seal lips facing toward the brake adjusters. That prevents grease from flowing onto the brake shoes as any excess will be directed away from the shoes.
Dust Shields: Dust shields help protect brake components from the environment and minimize the effect of brake fade while driving through water. If removed, dust shields always should be installed back on the vehicle's wheel-ends and they should be examined for tightness, corrosion or distortion.
Rollers: Brake shoe rollers ride on the S-cam and transfer the force from the cam head to the shoe. Flat spots on the roller will interrupt that movement and can cause a sliding motion that can damage the cam head. The trailing shoe drives the roller into the cam head (acts like the anchor pin on the leading shoe) causing more wear on that particular roller. Parts manufacturers recommend that the rollers be replaced with every brake job, you should replace any rollers that show any wear or flat spots. Oversized rollers are available to prevent cam-over if drum is oversize in diameter. Many manufacturers provide rollers in a series of sizes.
Anchor Pins: The anchor pins secure the shoes on the spider and allow them to pivot during an application. You should visually check the anchor pins for roundness or wear once the brakes are disassembled. Worn pins or a loose fit in the spider will cause uneven brake shoe wear. Stainless steel anchor pin bushings will be found on some spiders and should be replaced if they show signs of wear. Stainless steel bushings prevent corrosion and wear of the anchor pin hole in the spider.
Spiders: Check the brake spider for worn anchor pin holes, cracks and alignment. With single anchor pin spiders, you should check for anchor pin wear and looseness. If anchor pin looseness is found in the single pin design, a new spider assembly will have to be installed. With double pin design components, you probably only will need to replace the anchor pins and bushings. Bent spiders can cause tapered lining wear or cause brake shoes to extend past edge of drum.
Adjustment And Inspection: Improper brake adjustment is a major cause of premature lining and drum wear. With everything the same, such as chamber size, brake length, brake lining etc., the shortest push rod stroke will do more braking until it matches the next shortest, etc. This results in faster wear of the brake lining and higher temperatures on the wheel end with the shorter stroke.