Engine and Transmission Tech

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Cam Gear

The fabric camshaft gear camshaft drive of Ford secures simplicity and low cost of maintenance (as compared with the so-called silent chain drive) and yet secures silent operation at all speeds.

In actual practice it has been found the fabric gear is usually more durable than steel - possibly because there is a little "give" to it, which allows this gear to yield, rather than wear. This type of fabric gear drive has been used on the Buick, the Nash, and other large cars for many years. In fact, it is estimated that 69 per cent (more than two out of three) of the cars produced in 1929 were equipped with fabric timing gears, and 1930 will probably show an even larger percentage.

The advantages of the fabric gear include:
1. Permanent positive timing accuracy.
2. Nothing to stretch. Wear cannot cause lag.
3. No continuous adjustments or early replacements.
4. Remains quiet.

How Fabric Gears Are Made

The fabric camshaft gears, as used in the Model A Ford, are made from die-formed blanks. Sheets of canvas, impregnated with bakelite resin, are punched into rings, segments or discs. These are assembled to individual molds, and are then molded under pressure of 2000 pounds per square inch, while at a temperature of 1200 degrees Fahrenheit.

The web of the gear is made up of small segments of bakelite fabric which interlap and, when molded, give an interwoven web, which assures the gear against delamination because of web flex. The tooth surface of the blank is built up of rings and segments, which are punched from sheets of impregnated canvas, so the weave of the canvas radiates from the center of the gear. This assures uniform strength around the entire circumference of the gear. These segments and rings are interlapped with the segments used to build up the web of the gear, so the whole gear is interlocked when molded. The cut shows a magnified cross section of the gear, which illustrates the interwoven feature of the particular makes of fabric gear used in Ford cars.

When the fabric blanks come from the presses, they are given a special heat treatment lasting 48 hours at a temperature of about 1600 degrees Fahrenheit; which insures the gears against change of form when subjected to the hot oil in the crankcase.

All fabric gears are triple tested for strength and accuracy before they leave the factory. The first test for strength is given the gear blanks after they receive the heat treatment just described. This test consists of applying, at three places on the tooth surface of the blank, a pressure of 2000 pounds per square inch. Every blank must pass this test 100 per cent.

The second test is the flexibility of the web. A pressure of 2000 pounds is applied on the web, pushing it down 1/8 to 5/16 inch. To pass the test, the web of the gear must spring back into position simultaneously with the release of pressure.

The third test is actually a series of tests. These insure accuracy of the teeth, bore, and location of dowel pin hole. Less than0.0015-inch variation in the eccentricity of pitch diameter, less than 0.001 inch plus or minus variance in maintaining center distances of mating gears, and absolutely no variance in regular spacing of teeth are the standards of precision which every fabric gear must achieve before it is passed.

On a test fixture in a laboratory, a load of 2000 pounds was applied directly to the hub of a gear blank of the type being used on the Model A Ford at intervals of three seconds for three weeks. At each application of the load, the web deflected 1/8 inch. At the end of the test, no evidence of fatigue or loss of elasticity was noticed. It is apparent the ability of this gear to cushion or absorb shocks and vibrations not only tends to eliminate noise, but also reduces the wear and tear of engine parts to which it is connected.

The fabric gear is positioned on the camshaft by a dowel pin in the camshaft flange, and is held in place by a large round nut (which has two flat surfaces cut into it for the application of a wrench). Thus nut and flange form a metallic hub for the fabric gear. The fabric camshaft gear has 50 teeth, while the steel crankshaft gear that drives it has but 25 teeth. Consequently, the camshaft must rotate at the correct speed, which is one-half of crankshaft speed.