Gears.

Gears are devices, whose task is most often to reduce the rotational speed of the working machine in relation to the high speed of the drive motor shaft. In each transmission, we distinguish driving elements - active and driven - passive. The driver transmits the movement of the motor directly, or with the participation of intermediary elements, the driven element. Ratio of rotational speed n2 of driven wheel to rotational speed n, the driving wheel is called gear ratio:

where: D1, D2 - effective diameters of gear wheels,
z1 , z2 - number of gear teeth in the toothed transmission,
n1, n2 - rotational speeds of driving and driven element.

We distinguish between three types of gears: hovers, cable and gear.

Friction gears they transfer the rotational movement from one wheel to another thanks to the frictional force caused by pressing both wheels together - directly (picture a and b) or through the intermediate part (drawing c).

Friction gears: a) fixed ratio, b, c) continuously variable.

The gear in the drawing a has a constant ratio, the others are continuously variable. Variable ratio friction gears are used, among others. in. in confectionery mixers for whipping cakes.

Tension gears they transmit the rotational movement from one wheel to the other via the cable. Depending on the type of tendon, which may be a belt, rope or chain, a distinction is made between belt transmissions, rope and chain, and their components: pulleys, rope and chain. Pulley diameters are standardized.

Transmission gears - belt: a) Open, b) semi-open, c) crossed, d) with tensioner.

V-belts with a trapezoidal cross-section are used in many gastronomic machines. These belts consist of two layers: thin outer with high abrasion resistance and inner with high tensile and bending strength.

Cross-section of a pulley rim and trapezoidal V-belts.

V-belt transmissions are made like this, that you can change the track of the pulleys, which allows you to adjust the tension as the belt lengthens. The V-belts are arranged in the grooves of the pulleys.

As links of chain transmissions, ladder or toothed chains of various designs are used.

Chain transmission rods: a) ladder chains, b) ladder - sleeve, c) toothed gear with single and multiple chain

Toothed chains work better than ladder chains, because the design of the toothed links prevents the circumferential play that increases with the wear of the chain. Chain gears are used in cases, when the consistency of the gear ratio is desired and when the gearing cannot be used due to the considerable distance between the axes. Ladder chains and sprockets are standardized. Gear transmissions are used everywhere, where the distance between the driving and driven shaft is small.

Gear transmissions: a) parallel, b) angular, c) wichrowata, d) worm.

The figure shows some examples of gears. The gears of these gears may be cylindrical or conical; the worm gear consists of a worm wheel (scroll) and a worm usually made together with the shaft. The worm teeth are meshed simultaneously with several worm gear teeth, making it possible to transfer large moments of force, and the wear process of the teeth is slower than in other gears. Worm gears are used everywhere, where there is a need for a significant gear ratio, np. in cutter bowls.

The direction of rotation of the driven shaft in the gear transmission is opposite to the direction of movement of the driving shaft. If you do not want to change the direction of movement, it is between the gears of the mating shafts that the third idler is inserted (drawing b).

Directions of rotational speed of wheels in toothed gears.

Only one constant gear ratio is achieved with one pair of gears. To obtain several different speed of rotation of the driven shaft at a constant speed of the driving shaft, multi-stage gears are used. The figure shows a two-stage gearbox, in which the combined wheels z1 and z3 can be moved along the shaft. This allows the driven shaft to rotate at two different speeds: one, when the wheels z1 and z2 and the second one, when wheels z3 and z4 work together.

Diagram of a two-stage gearbox.

 

Exercise

Calculate the speed of rotation of the clutch, if the teeth numbers of the gears of the gears are: z1 = 18, z2 = 45, z3= 13, z4 = 44, and the shaft of the electric motor rotates at a speed n1 = 1450 RPM.