[Reconditioning Brake Drums]
[From the U.S. Government Publishing Office, www.gpo.gov]

RECONDITIONING BRAKE DRUMS
LIMITS AND TOLERANCES
Prepared for VEHICLE MAINTENANCE SECTION
DIVISION OF MOTOR TRANSPORT
OFFICE OF DEFENSE TRANSPORTATION..
by MAINTENANCE METHODS COORDINATING COMMITTEE OF TRANSPORTATION AND MAINTENANCE ACTIVITY SOCIETY OF AUTOMOTIVE ENGINEERS, INC.
GOVERNMENT PRINTING OFFICE - WASHINGTON - 1944
SAE Maintenance Methods Coordinating Committee
W. J. Cumming, Chairman, Chief, Maintenance Section Office of Defense Transportation
E. P. Gohn, Vice-Chairman, Automotive Test Engineer The Atlantic Refining Company
E. N. Hatch, Senior Mechanical Engineer New York City Transit System
M. E. Nuttila, Superintendent, Motor Vehicles Cities Service Oil Company
G. W. Laurie, Manager, Automotive Transportation Department The Atlantic Refining Company
S. B. Shaw, Automotive Engineer Pacific Gas & Electric Company W. A. Taussig, Automotive Engineer Burlington Transportation Company E. W. Templin, Automotive Engineer .••••	Los Angeles Bureau of Power & Light
D. K. Wilson, Superintendent, Automotive Equipment New York Power & Light Corporation
•	A. M. Wolf, Automotive Consultant
V. .Subcommittee on "Reconditioning Brake Drums,
•	Limits and Tolerances"
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Bruce Crane, Chairman, Field Representative, The Ethyl Corporation J. V. Bassett, Sales Engineer, Raybestos-Manhattan, Inc.
II. L. Debbink, Superintendent Gasoline Vehicles, Milwaukee Electric Railway & Transportation Company
J. R. Glazebrook, Technical Advisor, Johns-Manville Corporation
F. F. Green, Superintendent Equipment, Department of Public Works, Fresno J. C. Hubbard, Assistant Transportation Superintendent,
Kraft Cheese Company
C. K. Hunt, Sales and Service Engineer, Universal Auto Parts R. K. Super, Sales Engineer, Timken-Detroit Axle Company E. H. Wells, Jr., Chief Engineer, Johns-Manville Corporation
A. E. Wilson, Chassis Engineer, White Motor Company
W. A. Taussig, Project Chairman
Reconditioning Brake Drums, Limits and Tolerances
Motor vehicle brake drums are subject to a number of deteriorating factors which, if not corrected promptly, can impair the efficiency of brake operation and also cause premature failure of other parts. Heat checking, for example, causes excessive lining wear and, in addition, sometimes leads to cracking of the drum. A grooved drum, if used with new lining, will not only wear the lining but it will also make it very difficult, if not impossible, to obtain efficient brake performance. Out-of-round drums make accurate brake adjustments impossible and are likely to cause excessive wear on the moving parts of the entire braking system due to their eccentric action. Out-of-round brake drums can also cause severe and very irregular tire tread wear.
Since the condition of the brakes vitally affects the reliability and availability of the vehicle for service, it is the responsibility of every truck owner, not only from patriotic motives but also to guard his own interests, to carry out a REGULAR PERIODIC INSPECTION of the braking system as exemplified in the SAE-ODT report on “Preventive Maintenance and Inspection Procedure”. Brake drums must be inspected regularly since, without inspection and the necessary remedial work, incipient failures will progress to the point of actual part failures, causing interruptions in service'and requiring replacement of parts which, in many cases, are hard to obtain because manufacturing facilities have been converted to the production of war materials.
The inadvisability of using cracked or broken brake drums is obvious, but the Committee has convincing evidence that even drums in this condition, which heretofore have been almost always scrapped, actually can be repaired for many miles of additional service.
In order to maintain brake drums properly, the maintenance inspector must be able to recognize incipient failures and must understand the main-1 tenance required to prevent actual failures.
While instructions on brake drum maintenance have been singularly lacking heretofore, the Committee believes that the following information, based upon actual practices in truck maintenance
and repair shops, will prove valuable to any person responsible for the maintenance of automotive brake drums.
COMMON BRAKE DRUM DEFECTS
Heat Checks.—It appears to be impractical to attempt to measure the extent of heat checking and record the result in figures. As a guide for determining the necessity to recondition drums, the Committee recommends a study of illustrations, figures 1 to 5, which show various degrees of heat checking, and recommends that these illustrations be considered a guide in determining the necessity for reconditioning brake drums.
Grooving.—Grooving of brake drums can be remedied by remachining the braking surface, usually at the time of installing new brake blocks or linings. The procedure for avoiding grooved drums lies in the maintenance schedule of the vehicle owner. At the regular preventive maintenance inspections, the brake mechanism should be cleaned, lubricated, adjusted, or tightened, as necessary, and checked for worn or defective parts which should be repaired. At each inspection, the mechanic must determine whether or not the linings will last until the next inspection. If the brake lining thickness is not sufficient to last until the next inspection, the brake lining should be replaced and the brake drum should be inspected to determine whether or not reconditioning is necessary. If the lining does not need replacement, but is more than half worn, the drums should be cleaned but should not. be remachined. At this stage, eliminating the grooves, on the drum and smoothing the ridges on the lining would necessitate removal of too much metal and lining, while, if left alone, the grooves and ridges match and satisfactory service can be obtained.
If the linings are little worn and drum grooving is present, the drums may be remachined and the ridges in the lining may be removed with a lining grinder.
Brake lining thickness can be visually checked through peep holes in the drum or peep holes in the brake backing plate. In order to assist in this
604619°—44
(1)
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Figures 1 and 2.—M ild heat checking, covering a small portion of braking surface and preventing no outstanding sharp edges. No reconditioning needed
checking of brake lining thickness, some operators drill peep holes in the backing plates opposite points of greatest wear on the brake shoes. These peep holes are also of assistance in adjusting brake shoes.
Out-of-Round.—Out-of-roundness is a condition which may be measured in several ways and it is generally accepted that a drum more than 0.010 inch out-of-round on the diameter is unfit for service and will cause bad tire tread wear. Out-of-roundness can be removed by returning the brake drum on a lathe.
Figures 6 and 7 show methods of measuring the inside diameters of a drum by using an inside micrometer or a piece of welding rod and feeler gages. It is desirable to take these measurements at the open and closed edges of the drum and at right angles to each other. To determine the amount of bulge, it is necessary to take measurements at several points between the outer and inner edges of the drum.
In practical operation, out-of-roundness may be easily detected when making a regular brake adjustment. The brake shoes may be adjusted for slight drag, the wheel turned by hand and any increase or reduction in drag will indicate out-of-roundness. If the drum is out-of-round, the wheel should be removed and checked with measuring equipment for the degree of eccentricity. If, as outlined above, it is found to be 0.010 inch or more out-of-round, it should be reconditioned.
With air brake equipment, the mechanic can determine the degree of out-of-roundness of drums by checking the extent to which it is possible to obtain a standard brake adjustment. At regular brake inspections, brake shoes should be adjusted to specified standard measurements of travel of the cylinder piston or diaphragm rod. If, after making a standard adjustment, the wheel does not turn freely, as a result of the brake drum binding at spots, the drum is out-of-round. Under these conditions, a free wheel can be obtained only by
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Figure 3.—Borderline heat checking. No reconditioning needed yet, but careful inspection at frequent intervals is indicated.
Figures 4 and 5.—Severe heat checking. Reconditioning required for preservation of normal drum and lining life. Investigation to determine compatibility of drum and lining to type of service may be indicated.
slacking off the adjustment and, if the difference in adjustment between high and low spots for a free wheel is more than 25 percent of the standard adjustment, drum reconditioning is indicated.
Bell-Mouthed.—Bell-mouthing of drums is a condition seldom encountered but, if there is any bell-mouthing at all, the drum should be reconditioned.
RECONDITIONING METHODS
Drum Turning.—Brake drums, requiring reconditioning for any of the above reasons or because they have been welded, may be refinished by either turning or grinding or both and may be honed to give the smooth finish desired for best brakes and longest lining wear. There are several makes of grinders available, all of which are satisfactory if properly used. Turning may be done on a brake drum lathe, of which there are several makes, in
eluding portable equipment, or it may be done on a vertical boring mill.
When finish-machining brake drums, each drum should be mounted on the wheel hub with which the drum will be used and it is essential to select the proper cones for mounting on the lathe, in order to properly center the drum and insure concentric cuts with the tool. Figure 8 shows a properly fitting cone being placed into the hub for lathe mounting. A round-nosed tool is usually used in the removal of metal, at least on the first cut. The depth of the cut is determined by the operator after he has found the shallowest portion in the drum braking surface. Figure 9 shows a scored drum after a first cut of 0.030 inch on the radius (0.060 inch on the diameter) has been made, using a round-nosed tool. It is frequently necessary to make two cuts in order to remove the required amount of metal to get an even surface throughout the total width of the braking area. Figure 10
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Figure 5.
shows the position of the round-nosed tool for a second cut in order to remove all traces of the bowed and scored condition.
The speed with which the drum is turned and the feed of the tool are determined by considerations of drum material and type and power of the machine. The drum lathe manufacturer’s in-tructions should be followed.
For the finish-grind operation, one brake drum remachining equipment manufacturer recommends a drum speed of 60 revolutions per minute, a grinding wheel feed of 0.010 inch per revolution, and a grinding wheel surface velocity of 6,000 feet per minute. Figure 11 shows the drum surface after the finish-grind, taking a cut with the wheel of 0.002 inch on the radius (0.004 inch on the diameter).
Some operators, rather than remove metal in fixed amounts to match standard lining or shim oversizes, take skim cuts at certain inspection periods, generally 20,000 to 40,000 miles. This is
a simple operation and eliminates the necessity of taking heavy cuts. It also eliminates the rapid wear of lining which accompanies the partial destruction of the drum. In this way, complete preventive maintenance may be applied to brake drums.
BRAKE DRUM WEAR LIMITS
In turning or grinding material from brake drums to resurface them, the maximum amount of metal which may be removed is determined not only by considerations of safety but also by practical considerations of brake operation. For example, there are operators running heavy equipment who remove as much as 40 percent of the original (new) thickness of the brake drum wall before discarding the drum or building it up by welding. In actual practice, the amount by which the drum thickness may be reduced is limited by drum distortion which results in poorer braking and rapid lining wear and by the fact that most brake shoes are not adjustable on the anchor-pin end and, as a result, much of the lining in the larger oversizes cannot be worn away. Furthermore, many brake shoes cannot be expanded to more than .250-inch oversize diameter without mechanical interference. In cases where these difficulties do not apply, it is safe practice to remove up to 25
Figure 6.—Measuring drum with inside micrometer to determine out-of-round, bell mouthing, or bulge; 0.010 inch out-of-round indicates reconditioning.
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Figure 7.—Alternate method of measuring drum, using welding rod and feeler gage.
Figure 9.—First cut of 0.030 inch on radius (0.060 inch on diameter) has been made on this drum, using a round-nosed tool.
Figure 8.—It is essential to select the proper cone in turning drums to insure concentric cutting.
• Figure 10.—Position of round-nosed tool for second cut to remove all traces of irregularity. The depth of this cut may be up to 0.040 inch on radius (0.080 inch on the diameter).
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percent of original drum thickness on heavy-duty (truck-type) brake drums. This refers to drum body only. Ribs or flanges should not be considered part of drum for this measurement.
Figure 11.—Finished drum surface; 0.002 inch was removed on radius (0.004 inch on diameter) in finishing the drum.
A METHOD OF SALVAGING WORN-OUT DRUMS BY WELDING
The wearing surfaces of worn cast steel or pressed steel brakedrums have been salvaged by arc welding in the following manner:
1.	Set the drum up in a jig or on a welding bench in an upright position, so that welding can be done around drum and drum moved as welding proceeds.
2.	Use %6 or %2 inch mild steel electrode and acetylene rod similar to Oxweld No. 1, %6 or % inch. Feed the acetylene rod into the arc so as to put on more metal and speed up application. Put on enough metal so that the drum can be machined smooth.
3.	After welding has cooled, chuck drum in a lathe, using hole for the hub as center, and machine smooth.
4.	To finish-machine the drum braking surface, mount the drum on its wheel hub and finish-machine as outlined under section entitled “Drum Turning” on page 3.
A METHOD OF REPAIRING CRACKED BRAKE
DRUMS	;
Broken or cracked brake drums can be satisfactorily repaired by proper brazing and such repaired drums will give good performance. The technique for this type of repair is described below.
In brazing a broken brake drum, it is first necessary to get the drum as nearly round as possible, which can be accomplished by pulling it together with a chain puller, as shown in figure 12. This puller is used to hold the drum together while beveling the cracks. A length of chain around the drum with a screw jack will serve. If the drum is sufficiently oval to require straightening, it should be straightened in a press.
Figure 12.—V cut in drum flange, preparatory to welding.
With drum round and held together with the chain, saw or grind a V of about 100° nearly through the wall from the outside, if the drum is cast iron, or chamfer out with a torch to 75°-90° bevel, if the drum is steel. Figure 12 shows the V cut in the flange of a cast-iron drum ready for brazing. It is not necessary to leave the opening at the bottom of the V as wide as is shown in the illustration. To clamp the drum while brazing, the chain puller was removed and a length of chain and an old screw jack was substituted.
The cast-iron drum is preheated locally to a dull v« red with the torch before the brazing operation is I started. A good strong bronze, such as Oxweld No. 25 Manganese or similar rod, is used after tinning the V with a flux, such as Peterson’s No. 3	|
Special. Figure 13 shows the crack in the flange
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brazed and the crack in the periphery cut out ready to braze. Figure 14 shows the finished re-4} ) pair job ready for machining.
Figure 13.—Crack in flange has been welded. V cut in periphery is shown.
After the drum is brazed, it is advisable to reheat I the whole brazed and adjoining area to equalize shrinkage strains and allow to cool slowly, away from drafts. When the drum is cool it can be turned.
Cracks or breaks in the webs or back of the drum may be repaired by using ordinary brazing method.
Figure 14.—Weld has been completed. Drum is ready for machining.
ADAPTING BRAKE SHOES AND LINING TO RECONDITIONED DRUMS
Brake shoes should be generally inspected for worn bushings or anchor-pin holes, worn cams or cam followers. Badly worn parts should be repaired.
Each lined shoe must contact the drum surface throughout the entire lining length for best brake operation. Therefore, when oversizing lining for a reconditioned drum application, each shoe must have added to the original lining thickness one-half of the amount turned out of the drum diameter. This can be done by using oversize lining or by installing a brake shim of the correct thickness between the lining and the brake shoe and is
necessary in order to give accurate adjustment and full lining contact.
Care should be taken that brake linings and brake shims, where used, are installed on clean brake shoes, with all the rust and unevenness removed from the faces of the shoes. Lining and shim must fit the shoe tightly and should be applied so as not to allow greater than an 0.005 inch gage to be inserted between lining and shoe, figure 15.
Figure 15.—In applying lining to shoe, no greater than 0.005 inch space should be allowed.
With linings below % inch in thickness, an advantage may be gained through the use of cement in application. The cement used for this purpose is not affected by heat or moisture.
Lining above % inch in thickness is usually referred to as “brake block” and is generally furnished in segments secured to the shoe by bolts (with lock washers) or by rivets. When secured by rivets, care should be taken in drilling and countersinking the lining. When using drilled and countersunk lining sets, the riveting should be started in the center of the strip and continued in order toward each end of shoe as illustrated in figure 16. In some cases, the holes in the drilled and countersunk sets may not line up perfectly and it may be necessary to elongate some of the holes near the ends of the lining.
Brass is found to be the most satisfactory rivet material for fastening brake lining to shoes and is recommended for use whenever possible. Iron and steel are the strongest, but are abrasive to the drum surface. Therefore, they should be used only when great strength is necessary and, when used, care should be taken to insure against their touching
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Figure 16.—When installing rivet-applied lining, the riveting should start at the center and work toward the ends.
the drum surface. Copper and brass are next in strength, but copper acts abrasively and, though strong, should be avoided on steel brake drums.
Brake lining rivets should be of the semitubular type. The rivet shank should fit the hole and the solid body of the rivet should extend through the shoe, but no farther, figure 17. The tubular section
Figure 17.—Use correct rivet. The solid body of the rivet sljould extend through the shoe but no farther. Drawing shows cross sectional view of semitubular rivet.
of the rivet should protrude from the shoe a distance equal to the diameter of the rivet. The diameter of the rivet head should be at least 1/64 inch
Figure 18.—Use a roll set for riveting. A star set might split the tube; then the rivet would not fill the hole.
smaller than diameter of the hole drilled in the brake lining. A roll set should be used for riveting. (A star set might split the tube and the rivet would a| not fill the hole, figure 18.)
When lining is to be secured by bolts, it is generally furnished with 82° counterbored holes, although sometimes flat, counterbored holes are found. The proper degree bolt head must be used to insure the continued tightness of the applied lining.
Figure 19.—Place relined shoe in reconditioned drum and check with feeler gage. Clearance at no point should be greater than 0.004 inch.
A relined shoe should be carefully checked for proper arc and fit to the reconditioned drum with which it is to be used, as it is highly important to have the maximum area of contact between the friction material and the drum wearing surface. The arc may be checked by fitting the relined shoe in the drum, as shown in figure 19, and checking clearance at various points with a feeler gage. It is recommended that no greater than an 0.004 inch clearance be allowed between drum surface and the brake lining at any point.
Various methods are employed to fit the shoe to drum, such as grinding the lining surface or shaping the shoe in a shoe vise. The latter, however, cannot be done with cast shoes. There are different methods of grinding the lining surface. With all methods, however, the object is to obtain full lining contact without sacrificing any more lining thickness than is absolutely necessary.
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Brake block segments are manufactured in standard oversizes to accomodate reconditioned brake . ' drums. It is desirable to turn or grind the drum to '' fit one of the standard oversizes.
On brakes having adjustable anchor pins, the pins should be adjusted to give proper shoe heel clearance whenever drums are reconditioned. Final adjustments should be made to give maximum I lining-to-drum contact which tends to eliminate squeal, gives even lining wear and reduces pedal travel.
It is very important that the brake manufacturer’s recommendations as to adjustment techniques be followed closely in order to obtain best brake operation.
The recommendations of the brake lining manufacturers as to type of lining to be used for various operating conditions will be found most satisfactory, as a general rule. Their recommendations should be followed, unless unusual operating conditions result in poor brakes, aggravated cases of wear or defects of the type discussed at the beginning of this report.
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OTHER MAINTENANCE BOOKLETS
This ODT-SAE vehicle maintenance booklet is the ninth in a projected series. Thus far published are:
1.	Reconditioning Cast-In-Block Engines.
2.	Pistons to Fit Reconditioned Cylinders.
3.	Engine Bearings.	«
4.	Hard Surfacing.
5.	Preventive Maintenance.
6.	Cooling Systems.
7.	Mechanical Lacing (Cold Welding).
8.	Steering Maintenance.
Single copies of the booklets may be obtained without charge from the Office of Information, Office of Defense Transportation, Washington 25, D. C. Copies in quantity may be obtained from the Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C., at a cost of 5 cents each for Nos. 1 and 2, and 10 cents each for the others. A discount of 25 percent is given on orders for quantities of 100 or more.