New gears for old

SCRAM from UK based Scramgears uses an extendible belt, constructed so that for given constant input rpm at the drive point, the belt will loop past the drive point in a constant time however much the belt is extended.

In earlier times, water power in industrial processes did not require gearing for changing speed or torque. It was sufficient to allow more water onto the mill wheel, or increase the amount of steam into the piston.

But with the advent of internal combustion engines, which can only operate in a fixed range of rpm, it became necessary to introduce clutching mechanisms to disengage the output drive to start the engine. Gears also became necessary to provide a wider range of speed and torque.

These stepped gears employ pairs of gear wheels of differing diameters mounted on input and output shafts and arranged so that only one pair can mesh at the same time. The drawbacks to this system are the expense of cutting the gears, weight, lubrication and the necessity to disengage power to change gear.

To prevent shortening of gear life by crashing the gears while changing led to the development of synchromesh systems. The last important development in this type of gear has been the concept of automatic gears which detect requirements for increased torque and change the gear ratio automatically. The result is an expensive, heavy, inefficient and complex mechanism.

Developments in the textile industry where, for example, thread being wound onto a bobbin means that the winding speed of the bobbin increases as the bobbin is charged, leading to risk of breakage, concentrated on incremental speed control rather than the stepped control mentioned above.

From this arose the search for a practical continuously variable gear, whose ratio could be very gradually changed without disengaging the drive power. This concept is misleadingly called ‘an infinitely variable gear’ when ‘infinitesimally’ is meant.

Practical examples of this concept have been manufactured, such as the old Daf, or Variomatic, system, using a drive belt running over cones mounted on an output shaft. Moving the relative positions of the cones changes the effective circumference of the part of the cones that the belt runs on and so changes the ratio.

This system has problems of belt wear and energy efficiency. The Torotrak system is based on a 19th century invention and employs rollers interacting with the inner surface of a sphere. Both of these systems and indeed all previous ‘infinitely variable’ work essentially by arranging for an input drive wheel, either directly or through a belt, to act on variable diameters of a disk on the output drive (a cone or toroid being a three dimensional disk).

The drawbacks to these systems include the necessity of clutching, belt wear and slip because of the small area of contact between the circumference of the drive wheel and the surface of a disk.

Now there is an alternative.

SCRAM from UK based Scramgears uses an extendible belt, constructed so that for given constant input rpm at the drive point, the belt will loop past the drive point in a constant time however much the belt is extended.

One principal embodiment of this new invention employs a tension coil spring as the belt. This is driven round its endless path by lugs mounted within a hollow drive shaft and interacting between the windings of the coil.

Turning the shaft causes the lugs to propel the coil forward. It will be apparent that, as the number of windings of the coil does not change, for a given input rpm the belt will pass the drive point in a fixed time however much the belt is extended. However, the rpm of any wheel the belt runs over will increase if the belt is extended and decrease if the belt is shortened.

SCRAM as a reduction mechanism

Because SCRAM’s guide wheels have to be large enough to avoid over-straining the coil and wide spacing of the windings is not practical SCRAM acts as a reduction gear at ratios of between 15:1 and 25:1. This is extremely advantageous when employed with electric motors and turbines which have very high efficient r.p.m., generally in the 2000 to 6000 r.p.m range.

In order to provide practical rotation speeds most motors have to be reduced to speeds of 100 to 250 r.p.m. using a complex reduction gear. Common examples of such reduction can be found in worm gears in windscreen wiper motors and the motors that control electric car mirrors.

Such reducing gears have to be precisely and strongly built and installed to withstand the backlash of starting and stopping. SCRAM has no such problem and will have commercially valuable applications as a backlash and vibration free drive and as a reducing mechanism.

SCRAM as a speed control mechanism

The advantages of continuous speed control over a wide range do not need elaborating, but some minor aspects of speed control merit closer attention.

Many applications, such as video tape drives, require precise speed control within a narrow range. This is frequently achieved by inefficient means such as clutching, braking or increasing resistance in the electrical supply circuit This in turn means that a larger motor is employed than would be necessary if SCRAM was used.

Turning to wider ranges of speed in electric motor systems it is necessary to briefly discuss the problems of speed control in electric motors. Synchronous electric motors are limited in their speed by the frequency of the electric supply and the number of poles switched on in the motor.

Generally electric motors become increasing inefficient and generate heat if the load forces them to operate at less than their optimum speed. This in turn requires the system designer to specify motor power greater than necessary for the task, mostly to cope with start up load. This has expensive repercussions throughout the electrical supply system as start up power requirements are up to 10 times running requirements, so safety cutouts etc. are required and the capital costs escalate.

Obviously the cost of running a motor of higher power than necessary is also inefficient. The opportunity given by SCRAM to slowly accelerate the system to operational speed should provide great capital and running cost savings.

Direct current electric motors may have their speed controlled by resistors. As this entails turning power to problem heat it is clearly unsatisfactory. SCRAM could alleviate this problem.

Scramgears’ developers think that SCRAM will provide a useful mechanism for electromechanical systems designers to overcome some of the problems caused by the wide range of advantages and disadvantages of the numerous different types of electric motor.

More information is at: www.scramgears.com

Contact : John HammerbeckE-mail : john@SCRAMgears.com

Telephone +44 20 7589 3127Fax +44 20 7584 1944

SCRAM Technology Limited 2A Milner Street LONDON SW3 2PU