Helical gears or helical rack and pinion?

Posted on Aug 11, 2020

Helical gears or helical rack and pinion?

Gears are a key element of engines and machines drives; increase the output torque and control the direction of rotation or speed. Helical gears of plastic and metal gears in many shapes and sizes: spur, globoid, planetary and helical; with pinion, screw or different types of teeth. Among them, helical and spur gears are usually the most common in many industrial applications. Do you want to know their differences and features in depth? This article may help you.

Spur gears: advantages and disadvantages

Helical gears run smoother and quietly compared to spur gears. This difference is due to the oblique way their teeth interact about the axis of rotation. This axis may be parallel or generally crossed at an angle of 90 °. In the case of a crossover, helical gears are used together with a worm gear, although in these cases two bevel gears may alternatively be used.

When two teeth move, contact is gradual, starting at one end of the tooth and remaining in contact with the gear which rotates until full contact is achieved. A typical helix angle is from about 15 to 30 degrees. On the other hand, the longitudinal load changes in direct proportion to the size of the helix angle tangent.

Cross-axis helical gears: These are the simplest in the helical gear category. They create a screwing or wedging action as a result of high sliding along the sides of the tooth.

Parallel axis helical gears: consist of an infinite number of alternating gears of small thickness. This will cause each tooth to be skewed along the side as if it were a cylindrical helix. When in contact with each other, they should have the same helical angle, but in opposite directions.As a result of the angle of the helix, there is an axial thrust in addition to the load.

Double helical, or “herringbone”: They are a right-side and left-side helix combination. The axial thrust absorbed by the bearings of the helical gears is a disadvantage.

 

Advantages of helical gears

●     The angled teeth work more gradually, allowing for smoother and more silent gear operation when compared to spur gears or toothed wheels.

●     Helical gears last longer and are ideal for high-load applications, since they have a higher amount of teeth in contact.

●     Load is at all times distributed among several axes, which produces less wear.

●     They can transfer motion and power both among parallel and straight-angle axes.

 

Metal helical gears

 

Drawbacks

One of the disadvantages of these gears is the thrust along the axis of the gear which must be compensated by using appropriate thrust bearings. There is more sliding friction between the teeth. This causes increased wear during operation and the need for lubrication systems.

 

Helical gear efficiency is lower due to the contact between the teeth, which creates axial pressure and generates heat. More energy loss reduces performance.

Higher production costs than spur gears.

 

Gears: advantages and disadvantages

Spur gears or gears are the most common type of gears and are different from others because their teeth are aligned along parallel axes. They are used to achieve high gear ratios, low and medium speeds. Spur gears are very useful when movement needs to be transferred from one shaft to another that is parallel and close.

 

Advantages of spur gears

●     They can be used to transfer a significant amount of power (up to approximately 50.000 Kw).

●     They are very reliable.

●     This pieces are the simplest ones to design and manufacture. An actuation project that involves spur gears can be deployed faster.

●     They provide a constant and stable gear ratio.

 

The spur gear appears to be more efficient compared to a helical gear of the same size.

As the tooth is parallel to its axis, no axial force is generated. Therefore, the gear shafts can be easily mounted with ball bearings.

 

Plastic spur gear

●     These are low-speed gears.

●     They cannot transmit power between non-parallel axes.

●     Spur gears generate too much noise when operating at high speeds.

●     The teeth of the wheel are subject to high stress.

●     They cannot be used to transmit energy over long distances.

●     They are not as resilient as compared to other types of gears.

 

Regarding their applications, they are often used in sectors where slowing down is required and sound levels are not as important as when it comes to sales or the security systems industry. They also see an important application when transmitting motion in a planetary gear motor.

 

Rack and Pinion

Gear racks are used to transform rotary motion into linear motion. The rack has straight teeth cut into one face of a square or round section of the rod and mates with a pinion, which is a small cylindrical gear meshed with a rack. Generally, rack and pinion are collectively referred to as "rack and pinion". There are many ways to use gears.

To provide many varieties of rack and pinion, many types of racks are available in stock. If the application requires a long length that requires many racks connected in series, we have racks with correctly configured tooth edges. These are referred to as "machined end gear racks". When the rack is manufactured, the tooth cutting process and the heat treatment process can cause it to stop working. We can control this with special presses and repair processes.

There are applications where the rack is stationary while the rack is moving and others where the rack rotates around a fixed axis while the rack is moving. The former is widely used in transport systems, while the latter can be used in extrusion systems and lift / lower applications.

As a mechanical element transmitting rotary motion into linear motion, racks are often compared to ball screws. There are advantages and disadvantages to using racks instead of ball screws. The advantages of the toothed bar are its mechanical simplicity, high load-bearing capacity and no restrictions as to length, etc. However, the disadvantage is the play. The advantage of a ball screw is high precision and less play, while its disadvantage is that the length is limited due to deflection.

The rack and pinions are used for lifting mechanisms (vertical movement), horizontal movement, positioning mechanisms, stops and allow synchronous rotation of several shafts in general industrial machinery. On the other hand, they are also used in steering systems to change the direction of travel of cars. The characteristics of the rack and pinion steering systems are as follows: simple structure, high stiffness, small and light, and excellent responsiveness. Thanks to this mechanism, the pinion, mounted on the steering shaft, is meshed with the steering rack, which transmits the rotational movement to the sides (turning it into linear movement), so you can steer the wheel. In addition, racks and gears are used for various other purposes such as toys and side latches.

Spur gears are typically used in machines where actuations must operate at higher speed ranges (hospitality machinery) or lower noise levels (automotive or air conditioning industries). They are also commonly used on machines where high torque transmission along parallel axes is required.

:: Read more : Everything you need to know about Rack and Pinion Gears

 

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