Which gear better fit your needs? Helical or spur gears

A gear is the rotating machine part whose cut teeth, or cogs, mesh with another toothed part to transmit torque. They are a key component of motor and machinery actuators. They are used to increase the output torque and control the direction of rotation or the speed. There are many types and sizes of gears: plastic and metal gears in many shapes and sizes: spur gears, globoid gears, planetary gears and helical gears; with pinion, bolt, or different types of tooth. Among them, helical and spur gears are usually the most widespread across many industrial applications. This is why I want to explain the differences between them across this article in order you be able to choose the better fit for your needs.

Spur gears: the most used one

Spur gears or toothed wheels are the most common type of gear, and are differentiated from others since their teeth are set up along parallel axes. Their main use is to achieve large gear ratios, slow speeds and medium speeds. Spur gears are very useful when motion needs to be transferred from one shaft to another one that is parallel and close.

Beneficts of using spur gears

• They can be used to transfer a significant amount of power (up to approximately 50.000 Kw).
• They provide a constant and stable gear ratio.
• A spur gear tends to be more efficient when compared to a helical gear of the same size.
• This pieces are the simplest ones to design and manufacture. An actuation project that involves spur gears can be deployed faster.
• Since the tooth is parallel to its axis, no axial force is produced. Therefore, the gear shafts can easily be mounted with ball bearings.
• They are very reliable.

Handicaps of spur gears

• Spur gear are low-speed.
• The transfer is limited to parallel axes.
• The wheel’s teeth experience a great amount of stress.
• They cannot be used for transferring energy over long distances.
• They are less resilient than other kinds of gears.
• The noise produced by a spur gear when operated at high speeds is very high.

As for their applications, it is common to use them in sectors where a slow motion is required and sound levels are not as important, as is the case of vending or the security system industry. They also see significant use when transferring motion within a planetary gear motor.

Helical gears are generally used on machinery where actuations need to work at higher speed ranges (hotel & catering industry machinery) or lower sound levels (automotive or air conditioning industries). They are also commonly used on machines where high torque transmission along parallel axes is required.

It may interest you to read about speed reducers and how to improve their efficiency

Helical gears: why choose them

Helical gears operate more smoothly and silently when compared to spur gears. This difference is due to the oblique manner in which their teeth interact in relation to the rotational axis. This axis may be parallel, or generally crossed at 90°. In case it is crossed, helical gears are applied together with a worm gear, even though on these cases, two bevel gears may alternatively be used.

When two of the teeth move, contact is gradual, beginning at one end of the tooth, and staying in contact with the gear, which rotates until full contact is achieved. The typical helix angle ranges from about 15 to 30 degrees. On the other hand, the thrust load varies in direct proportion to the magnitude of the tangent of the helix angle.

Finally, going deeper into the available types, we can highlight:

• Crossed-axis helical gears: They are the simplest within the helical gear category. They produce a screwing or wedging action as a result of a high degree of slide along the sides of the tooth.
• 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.
• Parallel-axis helical gears: They are composed of an infinite number of low-thickness, staggered spur gears. This will result in each tooth being skewed along the side as if it were a cylindrical helix. When contacting 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.

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

Drawbacks of helical gears

• One of the disadvantages of these gears is the thrust which results along the gear axis, which needs to be accommodated by using adequate thrust bearings. There is a greater degree of sliding friction between the teeth. This produces greater wear during operation, and the need for lubrication systems.
• Helical gears have a higher manufacturing cost than spur gears.
• The helical gear efficiency is lower due to the contact between its teeth, which produces axial thrust and generates heat. A greater loss of energy reduces efficiency.

Did you know the strengths and weaknesses of these gears? If you need to purchase gears, at CLR we design and manufacture our own parts as per the highest European quality standards, and have a large online gear catalogue available. Whether plastic or metal, CLR’s injection and machining technology allows for the design of helical, globoid or spur gears to fit the needs of your project. No matter the part you requre, CLR can design and manufacture it with the utmost professionalism and dedication.