BONDING BENEFITS BRAKES
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ALTHOUGH bonded brake linings are coming into wider use for commercial vehicles, their advantages are not greatly appreciated. Probably the main attribute of bonded construction is that the wear life of the linings is not limited by rivet heads, which can protrude up to + in. from the surface of the shoe.
Relining of riveted shoes must be done before the rivet heads are exposed, if drum damage is to be avoided. Thus, for the 3-in.-thick linings of a typical, heavy vehicle, at least a third and possibly half the friction material is scrapped. With bonding this does not occur, and linings can continue in use down to a thickness of less than Yr; in. The actual dimension is often dictated by the adjustment available.
Records kept by a bus operator indicate that mileage obtained from Yr, in. of lining on vehicles in his fleet varied between. 3.000-12,000 miles, depending upon the material specified. The extra useful lining thickness obtained by bonding, could, in this case, represent 10.000-50,000 more miles between relinings.
parallel grooves around the inside of the drum. Bonded linings do not have these pockets, and so minimize
uneven wear of the drums and the necessity of frequent regrinding. Elimination of counterbores in the lining material also increases the
actual braking area by 6-12 per cent.
A lining measuring 4 in. by 16 in. may be attached by 20 rivets, each fitted into a bore 1 in. in diameter. This represents nearly 4 sq. in. of lost braking area per shoe, the equivalent of half a brake shoe on a fourwheeled vehicle.
Bonded linings tend to run cooler than riveted types as the entire undersurface of the lining is in close contact with the shoe. This improves the rate of heat dissipation. With (.12 large-scale reconditioning, brake shoes relined by bonding can be 20 per cent. cheaper to renovate than by riveting, whilst the cost of shoes and linings can be reduced by the elimination of drilling.
CIBA (A.R.L.), LTD., offer three adhesive systems for bonding friction materials which have to operate over a wide range of temperatures. The one most widely used by the motor industry is Redux 64. This is a modified phenol formaldehyde synthetic resin which gives reasonably good gap-filling and a bond of high strength at normal temperatures. It allows maximum operating temperatures at the bond line of 350° C., and has been used successfully on woven and moulded linings, and for disc-brake pads.
Redux 65 is of similar composition to 64, but can be used where operating temperatures are likely to exceed 400 C. Redux Liquid K6 and Powder C consist of a thermo-setting liquid synthetic resin and a thermoplastic powder resin. Used together the components give excellent gap-filling and exceptionally high bond strength at temperatures up to 90° C.
Bonding is a simple process provided that care is taken and instructions are accurately carried out. It can be divided into three main operations— preparation of parts, application of adhesive, and the final curing cycle.
The undersurface of the lining material must be free from grease, oil or any printing such as part numbers or makers' references. Printing can be removed with medium glasspaper or emery cloth, and dry-air blast applied to remove dust. Some manufacturers now supply linings for bonding which have no printing On the lower face.
Shoes or disc pads should be degreased in trichlorethylene vapour, and then shot-blasted on the,bond surface. It has been found that a pneumatic chisel is the best tool with which to remove the remains of old bonded linings. Degreasing and shot-blasting would follow their removal. If shotblasting equipment is not available, the shoe surface can be roughened by a powered abrasive belt or disc.
All the Redux liquid resins can be applied by brush, spray gun or roller. For spraying, Liquid K6 and 65 are thinned with industrial methylated spirit, whilst Redux 64 requires thinners made up from seven parts to three parts mixture by weight of industrial methylated spirit and ethylene dichloride. The thinners can be used as solvents for cleaning equipment.
Accelerating Evaporation Redux 64 and 65 are normally applied to both bond surfaces, following which the solvents in them must be allowed to evaporate. This can be accelerated by heating the cornponents in a ventilated oven for 30 minutes at 70° C. for 64, and 10 minutes at 100° C. for 65. If airdrying is employed, the components should be left overnight at-room temperatures.
Liquid K6 is applied thinly to both sarfaces arid, while the resin is still wet, these are dipped into a tray of powder resin, so that a layer of this material adheres to them. One hour at room temperature serves to evaporate the solvents, but this may be speeded up to 10 minutes if an oven at 70° C. is used. After the adhesives have .dried, the components for bonding may be stored. for long periods before curing.
The curing cycle, which requires the application of heat and pressure, must be carefully controlled. The pressure required varies from 50-100 p.s.i. and this must be maintained throughout the cure. For thick brake linings, such as those used on heavy vehicles, it may be necessary to exceed 100 p.s.i., to ensure that both bonding faces are brought into intimate contact with one .another.
The time-temperature cycle is important because the adhesive must be at curing temperature for the required period. Extra time should be allowed for the complete assembly to heat up.
Temperatures and curing times for the three Redux adhesives are given in the accompanying table. All times quoted are the minimum, and it is preferable to exceed them by 100 per cent. rather than reduce them by 10 per. cent. There' are many methods of applying the requisite heat and the one selected must depend on the type of assembly being bonded, the total number of components to be treated and the equipment already available.
Any oven capable of maintaining the curing temperature efficiently is satisfactory for this work, but it must be remembered that sufficient heat should be available to bring large masses of metal to the curing temperature. The simplest way of applying pressure to brake linings is to expand two similar assemblies inside a flexible steel band.•
For flat linings, such as clutch plates, a simple aligning jig may be employed with a pair of rigid steel plates forced together by a threaded metal rod. Oven-cured assemblies should be left under pressure while cooling takes place, although this is not essential for Redux 64 and 65.
Steam or electrically heated presses. are sometimes used for curing flat components, whilst heated steel anvils,. shaped to fit the inside radius of the shoe, can be employed for brake relining. The anvil is heated to 230280° C. and pressure is applied through a flexible steel strap by pneu
matic or hydraulic rams. By this means the cure can be effected in only 4 minutes.
It is essential after curing to ensure that even pressure has been applied. over the entire bond line. This is indicated by theextrusion of a bead of cured resin around the edge of the joint. Shearing off a small area of lining from one corner of the shoe and rejecting arty units which fall' below a predetermined standard are additional control of quality, whilst a percentage of all units bonded may be tested to destruction.
Brighter Future. . The bonded lining undoubtedly has, a bright future both for drum and disc brakes. Bonding is well able tostand up to the higher temperatures at which the brakes of current vehicles are expected to operate. In testing the bond strength of Redux 64 and 65 at elevated temperatures, CIBA found trials had to pe carried out with steelto-steel test strips, as the joints, effected between lining materials and steel were of sufficient strength to cause fracture of the lining material before the bond failed.
With linings bonded to aluminium alloy, the properties of the metal deteriorated to such an extent that the results obtained had no relation to the actual strength of the bond.