Spur Gears Vs. Helical Gears: Comparing the Pros, Cons, and Uses

Spur Gears Vs. Helical Gears
Spur and helical gears are two of the most common types of gears used in a number of machines. We shall learn about each of these gears, and compare their advantages, disadvantages, and applications.
Did You Know?
The whining noise that you hear when you put your car in reverse is due to the fact that, unlike the forward gears which are all helical, the reverse gear is a combination of three spur gears.
A gear is a rotating part of a machine. It has a number of teeth on it which are designed to mesh with the teeth on another similar gear having the same or different angle of orientation, thus allowing the transmission of power from one axis to another.

Usually, gears are used in combinations of two or more (known as gear trains), which are mounted on shafts and work in a sequential manner. In addition to allowing the transmission of power, they also help in changing the speed, direction, and even the torque of the rotation, thus allowing the machine to perform a variety of different functions.

Several types of gears have been developed over the decades, each having its own mechanical advantage as well as specific areas of application. Among these are the spur and the helical gears, which are commonly used in numerous domestic and industrial applications.

In the following lines, we shall find out what spur and helical gears are, and how they differ from each other.
What is a Spur Gear?
A spur gear is the most basic and commonly known type of gear. It is characterized by straight-cut teeth, which project out radially from a disk or cylindrical base, and are parallel to the gear axis.

Spur gears mesh together perfectly only if their axes of rotation are parallel to each other. For achieving a constant drive-ratio, instead of the basic straight-sided teeth, an involute teeth-profile is used.
According to the required load-bearing capacity, spur gears can be made from various types of materials, including cast iron, steel, brass, copper, plastic, wood, etc.
Pros
1) Spur gears provide a constant velocity ratio.

2) The teeth are made to be parallel to the gear axis (axis of rotation). Hence, these gears do not cause a thrust in the axial direction, and therefore can be easily mounted with the help of ball bearings.

3) These gears are easiest and cheapest to design and manufacture, and are highly reliable.

4) They are capable of transmitting significantly large amounts of power.

5) Compared to helical gears, spur gears are more efficient in transmitting power.
Cons
1) During operation, the entire face of each tooth meshes with the respective tooth of the other gear all at once, leading to the generation of noise. As the operation speed increases, the amount of noise too increases. Hence, spur gears are best suited for low-speed applications.

2) During operation, the teeth have to bear a large amount of stress.

3) Spur gears cannot be used for power transmission between non-parallel shafts.

4) These gears can only transmit power up to a small distance.
What is a Helical Gear?
Helical gears are visually distinguishable from spur gears, owing to their unique design geometry, which sets their teeth at an angle to the gear axis. The surface over which the teeth are carved is cylindrical, and therefore each tooth takes the form of the segment of a helix.
These gears can mesh with each other, both when their axes are parallel, as well as when they are at and angle to each other. Depending upon the load and the particular application, these too can be made using a variety of materials, such as steel, cast iron, brass, copper, plastic, etc.
Pros
1) In helical gears, as opposed to spur gears, the teeth engage a little at a time, instead of the entire face of each tooth at once. This allows for a silent operation.

2) Along with the transmission of power between parallel shafts, though at the cost of efficiency, these also allow power transmission between non-parallel shafts.

3) Since the teeth are diagonally positioned, they are larger in size as compared to the teeth in spur gears. Thus, for the same gear size, helical gears can handle more load comparatively.

4) Due their unique design, at any given time the load is distributed along several teeth which are in contact with each other. This significantly reduces wear and tear, and further increases the load-bearing capacity as well as the operating lifespan of these gears.
Cons
1) In helical gear trains, there is a sliding-type contact between two gears. This causes an unwanted thrust in the axial direction, and also leads to the generation of more heat due to sliding friction. Both of these result in the loss of the transmitted power, lowering the efficiency.

2) For addressing the unwanted axial thrust, special thrust bearings need to be used. Additionally, for reducing the sliding friction, additives need to be added to the lubricants. As such, a helical gear train design is more complicated, and the operating cost too is more.
Applications
Spur gears are used in many of the everyday machines. They are ideal for low-speed applications, which need a simple design and greater power-transmission efficiency. Also, since these gears have noisy operations, they should only be used in applications wherein noise isn't a major factor of consideration. Spur gears are used in washing machines, blenders, clothes dryers, mechanical clocks, aircraft engines, etc.

For applications that require a smooth and noiseless operation, and where some power transmission loss and lower efficiency is acceptable, helical gears can be the perfect choice. They are also most effectively used in high speed and greater load applications. Helical gears are used in the fertilizer industry, railways, automobiles, printing industry, earth moving industry, etc.