Spiral bevel gears are used in situations where the axis of rotation of a shaft must change direction. An example of this is a car differential where the rotating prop shaft is turned through 90° to drive the wheels.
Ideally this situation would transmit the required torque with maximum efficiency whilst producing minimum noise and vibration with a system that occupies minimum space at the required ratio reduction. The various bevel gear systems have their advantages and disadvantages and compromises are needed. An alternative to a bevel gear system, if the requirements allow, could be a worm drive (or worm gear set).
It is not a rule but bevel gear axes are commonly mounted at 90° to each other.
Straight Cut Bevel Gears
The axes of straight and spiral bevel gear systems, if extended, intersect i.e. their axes are on the same plane.
The intersect of a bevel gears pitch surface with its axis is called the pitch apex. With a mating gear pair, their individual pitch apexes should be made to intersect in order for appropriate teeth engagement.
If maximum efficiency is required, straight and spiral bevel gears are most appropriate because they transmit load through rolling contact of the teeth resulting in little loss through friction.
Straight cut bevels are however relatively noisy since the tooth flank does not contact gradually as per helical cut gears.
Straight cut bevel gears are generally used in low speed applications where gear ratios of 1:1 to 5:1 are suitable.
Bevel gears with shafts at 90° each other and with the same number of teeth (1:1 gear ratio) are called Mitre Gears.
Spiral Bevel Gears
A spiral bevel gear has teeth cut in helical form with the pitch surface being conical.
This results in a reduction in noise and vibration through the gradual engagement of spiral teeth with the contact starting at one end of a gear tooth and progressively spreading across the whole tooth flank.
Typical gear ratios are 1:1 to 5:1.
The axes of a spiral bevel system are on the same plane.
Straight cut and spiral bevel gears offer relatively high efficiency due to the rolling contact between gear teeth.
Hypoid Bevel Gears
These are similar in appearance to spiral bevel gears and again have a helical tooth form. However, the pitch surface is a hyperboloid and the axes of the gear system do not intersect. This axial offset places a hypoid gear set geometry between that of a worm and a bevel gear set and therefore displays a combination of their characteristics. I.e. like a worm gear set it can transmit a large torque and, like a bevel gear set, it can run very smoothly with little noise or vibration. The more closely it resembles a bevel gear set, the higher the efficiency due to less losses through a lower sliding contact between the teeth. The positioning of the hypoid gear implies that multiple teeth mesh simultaneously thus sharing the load over a larger surface area and allowing for a greater transfer of torque.
Hypoid gear systems are often used in the differential of a truck where the requirement for the transmission of a large torque suggests a worm gear set, but at the same time the requirement to maximise the efficiency and smoothness necessitates a spiral bevel gear.
With car differentials, due to the emphasis being placed on fuel economy and smoothness, there is an argument for the utilisation of spiral bevel gears. Practically however, the gear sizes required in order to transmit the required torque may result in too large a gear system that impedes on the required ground clearance and therefore a hypoid gear system is required.
Hypoid gears can offer large gear reductions with ratios in the range 3:1 to more than 400:1.