Supercharge - Chapter 8

Chapter 8 - Driving The Supercharger

The usual method of driving the supercharger back in the early days of racing was to direct couple it to the front of the crankshaft. It is still done with some of the 'drag' cars, or 'rails' in the States, but generally it is rather uncommon these days since it has only one advantage and several disadvantages. It is to its advantage that this method of driving involves no loss of power through transmission, but with the drive methods of today this is a small thing. Briefly its disadvantages are:

It takes more chassis room.
There is less weight concentration.
Longer manifold pipes are required;
It is impossible to vary the boost without changing the supercharger itself. Also mounting and correct alignment are engineering problems in themselves and even then some form of semi-flexible coupling must be used.

The last decade has seen very great improvements in belts as a form of power transmission. With the introduction of steel and rayon and nylon reinforced belts, both V and toothed belts are rapidly displacing chains as a form of power transmission. This is particulary so where high speeds are involved.

The Gilmer or toothed belts consists of a thin steel or nylon backed rubber or neoprene strip formed into an endless belt and having on the inner surface very shallow teeth for the whole of its length.

It runs on flat (no crown) very shallow toothed sprockets, usually of steel. Both belt and sprockets need to be of precision manufacture. If correctly made it is capable of transmitting up to about 20 b.h.p. per inch of belt width if operated at high speed.

Unfortunately at present in this country, Australia, it is not readily available, and at the time of writing must be ordered from Power Transmissions Ltd of Melbourne. It is not cheap. A one-inch belt of about 30 inches total length, and two sprockets, will leave little change out of sixty dollars.

This type of drive is exceedingly critical of alignment because the sprockets have no crowns. At least one of the sprockets must be provided with raised edges or sides about three eighths of an inch high. Without these it is impossible to keep the belt on the sprockets. This type of drive is almost universal on the big 'rails' in the States today.

Undoubtedly this drive has a very bright future. It has been used for some years to drive the overhead camshaft of a continental car and for a similar task in the United States. For a mechanical device to obtain a footing at all in the automotive field of today it must be good.
At present the most common type of drive for a supercharger is the V belt. These are so common that I will not describe them. They do however vary in size and design.

Most supercharger drives use industrial belts. These are measured over the whole length and are available in one-inch increments. They are basically of two sizes, 'A' and 'B' section. The former is a half-inch wide over its greatest width, and the latter five eighths wide.

The manufacturers of these belts give a somewhat conservative estimate of their power capabilities ( a good fault ). They list the maximum power transmission of the 'A' as 2 b.h.p. at a belt speed of 3000 f.p.m. and of the 'B' as 4 b.h.p. at the same speed.

The manufacturer quite rightly advises against the use of small diameter pulleys. My own experience confirms this. Pulleys should never be less than 4" O.D. and plenty of 'wrap around' should always be used.

The use of large diameter pulleys might appear to pose a problem. A 5" pulley at 6000 r.p.m. gives a belt speed of 7500 f.p.m. or just 2 times the manufacturers stated maximum. However, as I said previously, the manufacturer has been too modest, or else he doesn't know just how good his products are. Of course a belt operating at this speed can transmit twice the horse-power that it can at half the speed.

Now as to how many belts are required to drive a particular supercharger. This depends on the type of supercharger, its size and the amount of boost intended.

As a rough guide a standard car engine of 1500 c.c. using a seven pound supercharge and pulleys no smaller than 4 inches will require one 'A' section belt. A two litre with the same boost cam manage with one 'B' section. A three litre will need two 'A' section. And a five litre V8 will need three 'B' section. With all of these it can be said that the larger the pulley size the longer the belt life. In all cases belts must be kept tight. Belts slipping at high speed have a life of less than a minute. If they are not allowed to slip they will last up to 10,000 miles. These boosts can be exceeded somewhat if belts are very tight but belt life is reduced under these conditions.

'V' drives must be in reasonable alignment otherwise increased wear will occur with both belts and pulleys. Bad misalignment combined with tight belts will usually cause the belt to turn inside out and run on one of the outer corners. In this position the belt is much slacker. Once a belt has done this even for a few seconds it must be discarded as it becomes deformed and will repeat the process continuously.

There are several other types of 'V' belts on the market. Most of these I have not tried as they are available only in a limited number of lengths. One of these is a double V in section with two 'V's joined by a common back or outer strip about an eighth of an inch thick. This would no doubt be less prone to stretching since the back could be heavily reinforced. Also the double width would give much greater lateral rigidity and prevent belt 'side slap'.

There is a very deep section 'V' on the market of about three eighths width which the manufacturer claims is superior to the 'A' section industrial. This is known as the 'alpha belt'. Made by Fenner, it is a good space saver.

The tensioning of belts can sometimes be a problem. If the supercharger itself can be made to adjust all to the good. This allows almost 180 degrees of belt contact with both pulleys; however this is seldom possible. The alternative is a jockey-type tensioner operating either inside or outside the circle of the belt. If it is used inside the belt it will have to take the form of a third 'V' pulley. A flat pulley operating on the inside of the belt usually causes the latter to turn inside out. A flat pulley type tensioner is quite satisfactory on the back or outside of the belt in this respect but of course it is hard on the belt from a flexing point of view since it continually bends the belt backwards. However it does increase the arc of contact. Since belts seem to wear out on the sides with this type of work, before they are destroyed through over flexing, I think back tensioning is the better of the two methods. The tensioner itself of course should always be as large in diameter as possible. A twin row water pump ballrace makes an excellent bearing for this type of tensioner. An old pushbike pedal crank forms an excellent basis for the swinging bracket. Of course it is far too long and needs to be cut and an end welded on to clamp the end of the jockey pulley bearing. A second old crank can provide this clamp.

With regards to pulleys. If weight is not of vital importance, cast iron or steel are vastly superior to aluminium, wear wise. A worn pulley is very hard on the belt, and will reduce life considerably.

In comparing the merits of the tooth belt with the 'V' belt drive each has its good points. The tooth belt is of course more positive but this of course can be a disadvantage. I have seen tooth belts 'stripped' a number of times by a backfire into the supercharger even when there was a large relief valve. Even under these conditions pressures in the manifold can rise to 100 p.s.i. for an instant. This of course imposes an enormous load on the belt for a fraction of a second. Under these circumstances the 'V' drive simply slips.

To transmit a particular horsepower the tooth belt requires less space. Probably about 50% less. On the other hand it costs at least 70% more to buy, and the belts are not readily available. A change in the drive ratio would cost at least double that of a similar change with the 'V' system.

I have not yet mentioned chain drives because they are seldom used today for this type of work. A chain must operate in an oil bath to operate satisfactorily. This is difficult and expensive to install. Also if a chain is used some form of safety clutch must be provided since a bad backfire could bend the supercharger drive shaft. A chain is positive all right, but sometimes a shade too positive.

Supercharge!

0 to100 m.p.h. in 14 seconds in a HR Holden

by Eldred Norman

Chapter 8 - Driving The Supercharger