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ENSURING TYRE RELP BILITY AND EFFICIENCY
Page 90
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pNEUMATIC-TYRE production on the huge scale carried out by the Dunlop Co., Ltd., necessitates constant attention to detail. Every process of manufacture must be studied and maintained at its highest state of efficiency, and, in this connection, there are no more important departments than those dealing with the qualities of the materials employed and their methods of treatment, which come into the domain of the Technical Department under the control of which, also, are research and road-testing.
The departments concerned occupy premises which in themselves are as extensive as a works of fair size, whilst a large and highly qualified staff is constantly employed, some of the work even going on day and night. Chemical and other teges are carried out with every batch of material entering Fort Dunlop ; even. general supplies, such as soap, do not escape meticulous examination, and the laboratories are so well equipped that many manufactur,.-Ts of vehicles and components solicit and obtain the co-opetation of the company in trying out their respective products. , A striking example of this resulted, some years ago, in the provision of a large machine for testing wheel wobble on actual v chides. This is still in use to-day and has contributed appreciably towards the avoidance of this trouble.
The chemical laboratories might well be the envy of many colleges, whilst for practical testing, new machines are constantly being added, many of them partially or wholly designed by Dunlop experts.
One of the most important divisions is the textile room. This is double-walled with the temperature and humidity closely controlled. One instrument used here .can test single cotton hairs, and we were interested to learn that cotton is as strong as mild steel, section for section. To permit comparison, the cotton hairs are " cut to a length of 1 cm., and weighed in quantities of 100 on a balance .arm mounted on a quartz fibre, a mirror on the arm throwing a spot of light on to a scale. The cord used iii the construction of tyre casings is tested on a remarkable 'fatigue machine, which gives cmtinuous fluctuations in tension and bending, the weighted cord being wound backwards and forwards around a number of rollers.
The compounding division, which is responsible for the correct ingredients and quantities in the various mixtures, is equipped with miniature mills and Galenders. There is also a miniature curing plant. Amongst the interesting machines here is a pendulum bouncing machine which measures the rebound and penetration
of a striking ball. The hardness and resilience of various rubber blocks, under different conditions of load and heat can thus be tried out The penetration is measured by a micrometer, adjustments being simplified y the making or breaking of a weak electric current Iassing to earphones worn by the operator. A smaller touncing machine of this type can deal with tiny s3mples cut from tyres..
For many purposes rubber must be particularly resistant In abrasion; and to check this a sample is mOulded it the form of a wheel and driven at 600 r.p.m. in contact with a grinding wheel. This friction wheel is braked by a disc between the poles of an electro magnet.
Rubber is tested in much the same way as metals. One machine measures the stretch at -,x,r.stant load, the breaking stretch and the load which .causes it.
Of considerable interest is the fatigue-test punching machine, in which cylindrical pieces of rubber mounted vertically are squeezed almost flat three times a second. The number of compressions is "counted, and the temperatUro and hardness noted. The speed on this machine is controlled by a stroboscope. .
Mercury vapour lamps are utilized to give the equivalent of the sunlight in tropical countries and thus indiate any sun cracking. In some cases, outer covers awl :niter tubes are subjected to ageing tests, during which .3:ley are stored in hot ovens for long periods.
The checking of materials is not confined to goods as received, but is carried over various stages of production. From the main mixing mills. Lamson tubes convey cuttings from each batch of mixed rubber to a control -laboratory, and the batch is held hack for an hour until proved satisfactory. Each sample goes into a curing press for 15 minutes. It is then tested for stretch, tension and specific gravity. Fluids are made up at different gravities, and the samples of rubber are required to float or sink in these fluids according to the purposes for which they are to be employed. In case of failure, further tests are carried out on an Avery gravitometer.
The plasticity is given by a plastometer. The rubber is heated in water in a cylinder, into which is forced a piston; the rubber extrudes through a small hole in the base, the size of hole being altered to suit the grade. This is usually employed for masticated rubber.
Now we come to what we may term the more practical tests. In the test house is a remarkable collection of machines, which is supplemented externally by a fleet of cars and lorries.
Testing is carried out with two purposes in siew—to check current products and to keep ahead with improvements, whilst a large number of the products of other makers is dealt with.
When an improvement is incorporated the normal procedure is to test it out first on machines, this being a rapid, cheap and accurate method. Later it is tried on the road. The chief advantage in testing on machines is that exact conditions can be repeated as often as .required. Load, speed, pressure and temperature conditions can be kept constant, but it is impossible to early out a real tread-wear test on a machine.
The standard form of test machine is a 60-inadiameter plain metal drum, against which the tyre is pressed under a given load. The drum is run at ar.y required speed, and by altering the load, pressures, etc., the operator can reproduce any form of -failure, such as of the bead, wall, tread, etc. If the test is to be for loosening of the tread of the casing, four metal obstacles, in. thick, are bolted to the drum. These give the tyre four heavy blows at every revolution of the drum. One of these machines tests the
largest giant tyres up to loads of 10 tons, as it is usual to subject a tyre under test to double its normal load.
The surface speed for large tyres varies from 15 to 30 m.p.h., but is now being increased, in some instances, to 40-50 m.p.h. Racing tyres are run under tread pressure in an armoured chamber at speeds up to 305 m.p.h. This explains why Sir Malcolm Campbell was able to rely upon his Dunlop tyres when he achieved the world's speed record. The 4-ft. 6-in, cast-steel drum used runs at over 2,000 r.pan., whilst the tyre makes nearly 3,000 r.p.m. To keep the drum in condition it is tempered at intervals.
In another machine two tyres can be mounted, one at n34
each side of a hcuizOntal: belt, so that a flat running surface is reproduced. This is iMporta.nt in connection with certain tests where a curved running surface would not be satisfactory.
If, while on The test machine, a tyre develops a small blister, this cceches p. trip, which automatically sounds
an electric alarm; similarly, the alarm sounds if the tyre deflates.
The temperature Within the thickness of the rubber often has to be ascertained. A neat device to make comparisons consists of an hypodermic needle, into which is inserted a rod of another metal, the two being soldered at the point end, thus constituting a thermo-couple. The warmth causes a small current to flow to a galvanometer.
An Avery impact machine drops a " tupp," weighing 55 lb., from any height required, and the tupp carries individual tools of various shapes and lengths, some being built up of steel laminations. An automatic pawl prevents the tupp from bouncing and giving a second blow. A trip device indicates when the tool has entered the tyre to its full depth. The purpose of this machine is to reproduce the type of damage caused when a tyre strikes an obstacle, such as a kerbstone.
The nearest approach to real service conditions is given in the test house by a rotary track machine, in which the tyre is driven over a circular track made of uneven granite blocks, the sides being raised so that the track may be covered with loose stones, small flints, etc.
The road tests are carried out on all classes of vehicle, the smallest type in the present car fleet being a 9 h.p. sports model, the largest a 24 h.p. lanclaulet, whilst on the heavy side 'there is a large passenger chassis running as a special van with a much smaller tyre size than normal, ahd sometimes carrying a load on the roof to simulate a double-deck bus ; then there are a 3-ton and two 4-ton lorries. The lorries do normal transport work during the day, whilst at night they are run under test, carrying the next day's test loads, each covering 1,600 miles per week. The cars are run in three eight-hour shifts, thus reaching 20,000 miles in eight weeks, which represents three years' work for many privately owned vehicles. Braking and skidding tests are carried out on a smooth asphalt surface. For skidding tests this is merely watered, because if it were greased the conditions at different times would vary so greatly.
We have been able to touch only lightly on some of the many aspects of Dunlop activity, but its thoroughness cannot he disputed.