Open Gearing / Gears

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Gears are grouped into five design categories; spur, helical, bevel, hypoid, and worm. They are also classified by the mounting orientation, either parallel or at an angle. Shaft orientation, efficiency, and speed determine the type of design to use in any given application.

Open Gear Types

Spur

Spur gears, or straight-cut gears, are the simplest and most widely used type of gear due to low cost, lack of end thrust, and low maintenance. The edge of each tooth is straight and parallel to the axis of rotation. The tooth shape is based on an involute tooth form, which produces a rolling action rather than a sliding contact between teeth. They mesh correctly only if fitted to parallel shafts. Proper lubrication is important to avoid excessive wear. Spur gears produce reaction loads only radially to the shaft and do not produce axial thrust loads.

They are used in slow to moderate speed applications like mill drives, hoisting equipment, washing machines, and screwdrivers. At high speeds, they are noisy and wear quickly. Pinions come with or without a hub, and gears are available with solid, webbed, or spoked bodies. Normal ratio range is 1:1 to 6:1.

Helical

A crossed-axis helical gear can be used when a design requires shafts to transmit power at different angles. They are best used for light to moderate loads and require the correct type of lubrication. Helical gears with 45 degree helix angles are widely used and can be positioned on either parallel or perpendicular shafts.

Worm

Used on right angles on non-intersecting shafts and consist of a cylindrical worm that meshes with a larger gear, often called a wheel. The worm has a screw-type thread and requires several revolutions to drive the wheel through a single revolution; so a wide range of speed-reduction ratios can be obtained in a limited space. They are suitable for applications with shock loads. The screw action results in quiet operation but worm gears exhibit lower efficiency than other gear types. There are three types of configurations:

1. The least common has a cylindrical worm meshing with a helical gear.
2. Single-enveloping has a gear with curved teeth that wraps partially around the worm.
3. Double-enveloping has curved teeth as well, but has a concave-shaped worm that wraps partially around the gear. This type has the highest load capacity and shock resistance.

Bevel

Transmit power between shafts with axes that intersect at an angle. These gears have teeth cut on an angular or conical surface, and typically have a 20 degree pressure angle. They produce a thrust load on both shafts. There are four basic types of configurations:

1. Straight: Most common. Has a conical pitch surface with straight teeth that taper toward the apex. When shafts are at right angles and turning at the same speed, the two mating gears are alike and called miter gears.
2. Spiral: Have curved teeth at an angle for smooth contact. Capable of higher torque loads and speeds when compared to straight bevel gears.
3. Zerol: Similar to spiral bevel, but their teeth are curved lengthwise. They generally have zero degree spiral angles.
4. Skew Tooth: Similar to the spiral bevel except the teeth are not curved; they are cut straight and at an angle to the shaft centerline.

Hypoid

Resemble spiral bevel gears and operate on shafts with centerlines that don’t intersect. Therefore, both shafts can be supported by bearings at both ends. To compensate for the offset shaft, the pitch surfaces are hyperboloids of revolution. They accommodate heavier load transmission and operate smoother and quieter than spiral gears. However, high contact pressures and sliding speeds require the use of special lubricants.

Face

Also called crown gear, the axis of each tooth lies in a plane that is perpendicular to the shaft axis. The mating pinion is either a spur or helical gear. The pinion and face gear are usually oriented at 90 degrees to each other. They have a smaller load capacity than bevel gears and are not common in industrial applications.

Traction Drives

Traction drives are like gears and transmit power through mating rollers (cones, cylinders, discs, rings, or spheres). Dry and lubricated types are available. These are used in place of belts, gears, or chain drives where space is limited. Advantages include quiet operation, high speed reduction, low vibration, and high efficiency.

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Content on this page was created using excerpts from the Power Transmission Handbook (5th Edition), which is written and sold by the Power Transmission Distributor’s Association (PTDA).

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In the introduction to our gears series we wrote about Backlash and Gear Ratios. Now that you have more background information on gears we can break down the five gear types and some advantages and disadvantages. Let’s start with the most commonly used gear: the Spur Gear.

Spur Gear

• Efficient andeasy to assemble
• Straight teeth that are easy to align
• Minimal power lost due to slippage

•  Loud at high speeds

• Must be used in parallel
• Not as strong as other gears

The advantages of using spur gears in your application may be obvious. Efficiency and ease of assembly are going to save you time and keep downtime to a minimum, but what about the disadvantages? For example, the noise at high speeds is due to backlash and the instantaneous force the teeth of the gears experience when they first engage. The force can lead to wear and tear over time, which can decrease the efficiency of the gears.

A possible solution to this problem is plastic spur gears which are useful in lightweight applications and applications where noise must be kept at a minimum. Similar to the plastic spur gear, a bonded plastic spur gear with a carbon steel core and MC Nylon teeth helps keep noise levels down as well.

If you are looking for more details, kindly visit Cylindrical Gear.

Another thing to consider is that spur gears can only engage properly if they are used in parallel, due to their straight tooth profile and location of the bore. In the example below, two spur gears are being used to slide a work piece from the right to the left.

The Bearing Built-In gear type allows you to synchronize the direction of rotation between two opposing gears as shown in the picture. Another special type of gear is the keyless spur gear. The keyless spur gear is available in three different types: E Type, F Type & G Type.

• E Type:  Keyless spur gear has a similar shape to the A type gear but has a bushing fastened to the outside of the gear.
• F Type:  Has the same shape as the A Type gear, but with a bushing attached to the inside of the gear’s width.
• G Type:  Keyless spur gear is similar to our B Type gear, but has a bushing attached to the outside of the gear.

Helical Gear

• Quiet & Smooth
• Can be mounted in parallel or crossed

•  

  Less efficient than Spur Gears

• Power loss due to slippage

Helical gears have a diagonal tooth profile which allows them to be quieter and smoother then spur gears, since the teeth engage more gradually.  Helical gears have the ability to be mounted in Parallel or Crossed. When the gears are crossed, you must select Same Handed Gears, Both Right, or Both Left. Some of the disadvantages of helical gears are they are less efficient then spur gears due to tooth slippage. The diagonal profile of the teeth is what causes the slippage and also leads to axial thrust on the shaft. It is important to choose a bearing that can withstand the axial thrust generated by the gears.

Bevel

Bevel gears are used for intersecting shafts and have a changeable operating angle due to their shape. Some of the disadvantages of bevel a gear is they are difficult to assemble due to the changeable operating angle. The shafts also experience a large force, so like helical gears, it is important to ensure the bearing can withstand the force. Depending on the manufacturer, Bevel Gears can be found in two types; Straight Type and Spiral Type.

• Straight Type:  Similar to spur gears and prone to suffering from backlash and excessive noise.
• Spiral Type:  Similar to helical gears due to their similar tooth orientations, quieter than the spiral but prone to tooth slippage.

*It is important to note that spiral bevel gears are only guaranteed to mesh with a gear from the same manufacturer and they should always be bought in pairs. 

Rack & Worm

Another type of gear is the Rack Gear and can be found in two different styles; the Flat Type and the Round Type. The advantages of rack gears are when paired with a spur gear or pinion; they can transfer rotary motion into linear motion. A disadvantage is they cannot run continuously, since the rack will eventually end.

A nice feature of flat rack gears is the number of mounting hole orientations.  Depending on how you plan on mounting them you can either use side tapped holes, bottom tapped holes or side counterbored holes.

Worm gears are composed of two pieces, the Worm Wheel and the Worm Gear. Worm gears are self-locking and quiet, but suffer from high power loss and high thrust load on the Worm.

Regardless of your design requirements, remember that there are a variety of gears available, each having unique benefits. Additionally, there are solutions for linear motion as well as rotary motion.  Hopefully, this break down of gear types has you thinking about new application designs or how to solve problems in existing ones. In the meantime, keep an eye out for part 3 of this series!

Can’t wait until part 3 of this series, or have specific questions you would like to have answered? Comment below, or visit us at www.misumiusa.com!

For more information, please visit Mining Gears.