Qingdao Vanhon Machinery Technology Co.,Ltd.

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Introduction to racks, spur and helical gears
Apr 06, 2023


The rack allows to convert the rotational motion of the spur gear into the linear motion of the rack.


While a traditional spur gear produces only rotational motion, a rack can produce linear motion. In a rack, the teeth are no longer arranged on the circumference of a cylinder, but along a straight rod (rack). The mating gear of a rack is always a spur gear.


A rack can be thought of as a tooth that spreads out from a spur gear. In principle, the rack can also be regarded as a cylindrical gear with an infinite diameter. In this respect, rack gears are just a limiting case of spur gears.


The tooth profile of an involute gear is convex on the outside (external curvature) and concave on the inside (inner curvature), while a rack has straight teeth (no curvature).


The toothed rack corresponds in principle to a spur gear with an infinitely large diameter. An involute rack and pinion has a straight tooth profile.


A transmission that converts rotary motion to linear motion through a spur gear (called a pinion) and a rack is also called a rack. Such rack drives are used, for example, in machine tools for moving machine slides.


Introduction to racks, spur and helical gears(图1)

Spur gear

Spur gears (spur gears) are the simplest type of gears. In a spur gear, up to 3 teeth mesh simultaneously.


If the teeth of a toothed gear run in a straight line, that is, in the direction of the axis of rotation, it is called a spur gear or spur gear. Such teeth can be produced very economically by hobbing, planing or shaping.


For spur gears, up to three teeth mesh with each other at the same time. However, at least one tooth must always mesh with a mating gear to ensure continuous power transmission. The more teeth meshing simultaneously, the lower the load per tooth and the higher the power that can be transmitted.


Since spur gears engage the entire width of the teeth at the beginning of the engagement, the force transmission also starts abruptly and is abruptly interrupted at the end of the engagement. This results in a relatively high noise level. Therefore, spur gears are only suitable for low peripheral speed applications.


Spur gears are the simplest and therefore most cost-effective type of spur gear! Spur gears do not allow transmission of excessive torque and speed.


Higher speeds and torques can be achieved using the helical teeth described below.


Helical gears


For the gears mentioned earlier, the axes of rotation are always parallel when meshing. Using a special variant of helical gears, gears can also be manufactured in such a way that the shafts run obliquely, that is, they cross each other and not intersect. In this case, one speaks of so-called helical gears or crossed helical gears (hypoid gears). Usually the axes of the paired helical gears run at an angle of 90°, but in principle any other angle is also possible.


Helical gears or crossed helical gears allow helical fits of gear shafts!



While helical gears must have the same helix angle (but different direction of rotation) when paired. As the name suggests, helical gears no longer have a pure rolling motion when meshing, but a helical motion. A typical helical movement is a permanent slide on the side. Therefore, there is no problem on the reference body of the crossed helical gear, and points of a pure rolling process can be assigned (ie the peripheral speed of the gear is not the same at any point). The reference body of the helical gear is no longer a "pitch body", but a so-called hyperboloid of revolution! A hyperboloid is obtained by rotating an oblique line about an axis of rotation.



The constant sliding of the sides usually requires special lubrication of the helical gears (hypoid gear oil), otherwise it increases wear. Due to the helical process of the teeth, the tooth flanks no longer have a linear contact but a point contact (exception: worm gears). Furthermore, the helical tooth path generates strong lateral forces, which must be absorbed constructively by suitable bearings.


Therefore, helical gears are designed for moderate torques and speeds, such as drives for machine tools. The use of helical gears also has a detrimental effect on transmission efficiency, which is less efficient due to the sliding process on the sides.


In addition to the already mentioned inclined arrangement of the gear shafts, the advantage of crossed helical gears lies in their low-noise operation. In addition, helical gears allow axial movement over a relatively wide range without too much negative effect on power transmission.


Helical gears for low noise emissions in the medium load and speed range!


It is also said to be a hypoid gear when mating a helical gear designed as a "cylindrical" helical gear. However, the reference shape for helical gears can also be "conical" (see the previous article on bevel gears). Such spiral bevel gears are also known as hypoid gears.


A special case of helical gears are so-called worm gears. Compared to the general case of helical gears, worm gears provide a linear contact of the tooth surfaces and thus can transmit higher torques.

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