Transmission between two non-parallel shafts, even with moderate loads, has many problems.Two helical gears are meshed with each other to obtain cross-shaft transmission. For example, two right helical gears of I5 degrees will provide a 30-degree angle of cross-shaft transmission.However, since it is a point contact rather than a line contact, the load will be limited.The effect of the shaft intersection is to superimpose a high relative ramp velocity at the point of contact, much higher than the sliding velocity normal to the parallel axis, which will lead to an increased likelihood of oil film breakage and gear tooth wear.
Bevel gears are used for transmission between two intersecting shafts, most often between vertical shafts.Spur bevel gears are bevel gears similar to spur gears that mesh in a similar manner.This gear is very sensitive to assembly errors because the distance between the second gear and the intersection of the axes must be ensured.Spur bevel gears are difficult to manufacture accurately and can only be verified on the transmission error device, so they are never used where high speeds and precise transmissions are required.
Spiral bevel gears are bevel gears similar to cylindrical helical gears. Although still relatively "rough", they can provide a smoother transmission than straight bevel gears. Further development of hypoid bevel gears used in automobiles It came out of the need to lower the center of the driveshaft, thereby reducing the height of the aisle typically used in rear-wheel drive vehicles.The hypoid bevel gear is 90 degrees between the shafts, but it is specially offset by 50mm, which improves the position of the rear axle, at the cost of increasing the sliding speed of the contact point.
The geometry of hypoid bevel gears is extremely complex, which is best left to the experts.Specially manufactured gears are extremely expensive due to their high installation costs, so right-angle drive designs must be avoided unless a standardized automotive rear axle drive can be used.Standardized devices are mass-produced, and thus employ well-developed expertise, so are extremely cheap and surprisingly accurate.For example, a well-made rear axle can transmit 5000kw power at 100r/min, and the transmission at the gear frequency can be accurate to within 5mm. Except for the ground parallel shaft helical gear, it is as accurate as all other gears.
Worms and worm gears have the advantage of being compact and capable of providing high reduction ratios, but with high sliding speeds.One of the transmission elements, such as a worm gear, is often fabricated from a material that can withstand poor lubrication conditions, such as phosphoric acid.This is because it is difficult to design and manufacture with sufficient precision to have a proper oil film between the mating surfaces during operation under load.The poor lubrication state caused by the poor oil film will cause local wear on the acute side until the surface shape is suitable.Unfortunately changes in temperature and load may require a different shape and further wear occurs.
Unlike involute parallel shaft gears, bevel gears and worm drives are sensitive to positional errors and must be installed with extreme care.Even a well-cut pair of gears can produce large errors in the frequency (tooth frequency) corresponding to once per tooth if there is an alignment or position error during installation.If high accuracy is required, only the transmission error test can be used, and it is better to have a method to adjust the shaft position on site.In those places where high transmission accuracy is required, bevel gears are generally used only on high-speed shafts, and then helical gears are used to reduce the speed to reduce the impact of bevel gear manufacturing inaccuracy.Worm gears can be precise, but they have to be extremely careful.