What’s the Difference Between Spur, Helical, Bevel, and Worm Gears?

by Carlos Gonzalez

Gears are a crucial part of many motors and machines. Gears help increase torque output by providing gear reduction and they adjust the direction of rotation like the shaft to the rear wheels of automotive vehicles. Here are some basic types of gears and how they are different from each other.

Spur Gears

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Spur Gear

Spur gears are mounted in series on parallel shafts to achieve large gear reductions.

The most common gears are spur gears and are used in series for large gear reductions. The teeth on spur gears are straight and are mounted in parallel on different shafts. Spur gears are used in washing machines, screwdrivers, windup alarm clocks, and other devices. These are particularly loud, due to the gear tooth engaging and colliding. Each impact makes loud noises and causes vibration, which is why spur gears are not used in machinery like cars. A normal gear ratio range is 1:1 to 6:1.

Helical Gears

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Helical Gear

Helical gears have a smoother operation due to the angle twist creating instant contact with the gear teeth.

Helical gears operate more smoothly and quietly compared to spur gears due to the way the teeth interact. The teeth on a helical gear cut at an angle to the face of the gear. When two of the teeth start to engage, the contact is gradual–starting at one end of the tooth and maintaining contact as the gear rotates into full engagement. The typical range of the helix angle is about 15 to 30 deg. The thrust load varies directly with the magnitude of tangent of helix angle. Helical is the most commonly used gear in transmissions. They also generate large amounts of thrust and use bearings to help support the thrust load. Helical gears can be used to adjust the rotation angle by 90 deg. when mounted on perpendicular shafts. Its normal gear ratio range is 3:2 to 10:1.

Bevel Gears

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Bevel Gears

The image above shows two different configurations for bevel gears: straight and spiral teeth.

Bevel gears are used to change the direction of a shaft’s rotation. Bevel gears have teeth that are available in straight, spiral, or hypoid shape. Straight teeth have similar characteristics to spur gears and also have a large impact when engaged. Like spur gears, the normal gear ratio range for straight bevel gears is 3:2 to 5:1.

Spiral teeth operate the same as helical gears. They produce less vibration and noise when compared to straight teeth. The right hand of the spiral bevel is the outer half of the tooth, inclined to travel in the clockwise direction from the axial plane. The left hand of the spiral bevel travels in the counterclockwise direction. The normal gear ratio range is 3:2 to 4:1.

Hypoid gears are a type of spiral gear in which the shape is a revolved hyperboloid instead of conical shape. The hypoid gear places the pinion off-axis to the ring gear or crown wheel. This allows the pinion to be larger in diameter and provide more contact area.

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Pinion & Gear

The pinion and gear are often always opposite hand and the spiral angle of the pinion is usually larger then the angle of the gear. Hypoid gears are used in power transmissions due to their large gear ratios. The normal gear ratio range is 10:1 to 200:1.

Worm gears are used in large gear reductions. Gear ratio ranges of 5:1 to 300:1 are typical. The setup is designed so that the worm can turn the gear, but the gear cannot turn the worm. The angle of the worm is shallow and as a result the gear is held in place due to the friction between the two. The gear is found in applications such as conveyor systems in which the locking feature can act as a brake or an emergency stop.

 

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5 Common Motor Myths

by Stephen Mraz

Electric motors are incredibly common in manufacturing and many engineers are well-versed in their operation and principles. But the average consumer, and even a few non-electrical engineers are unaware that what they think is true about motors and efficiency just isn’t true. Here are five of the more common myths about motors.

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Baldor 1-Phase AC Motors

1. Higher temperatures have little effect on electric motors. Properly designed motors fall into specific insulation classes. The class determines the motor’s maximum operating ambient temperature rating. That rating, which includes some level of load, accounts for the threshold temperature the motor should remain beneath. For each 18°F this threshold is exceeded, the motor’s life is cut in half.​

2. Frequent startups do not hurt a motor. If a motor is not designed for frequent starts, then subjecting it to them will shorten its operational life. The initial rush of starting current generates extra heat, which usually dissipates while the motor runs. But if the motor does not run long enough between starts, there’s no time to shed the extra heat and the motor could exceed its maximum operating temperature.

3. Power-factor corrections save a lot of energy. Power-factor correction can reduce energy use, but only by a small amount. So unless your utility requires power-factor correction or charges penalties for low power factors, improving it will not affect your electric bill. The amount of energy saved depends on several site-specific factors, including the mix of electrical loads connected to your meter, the type and length of conductors, and where power-factor-correction equipment is placed (i.e., closer to the meter or closer to the motor loads). However, even in extreme cases, it is unusual for electrical consumption savings to be greater than 2%.

4. High-efficiency motors save more energy than standard-efficiency motors. In fact, an induction motor’s operating speed is somewhat less than its synchronous speed. The motor turns at the synchronous speed if the motor shaft’s rotation matches the frequency of the ac electricity powering the motor. The difference between synchronous and actual speed is called “slip.” Many energy-efficient motors operate with less full-load slip or at slightly higher speeds than comparably sized efficiency motors.

For centrifugal fans and pumps, even minor changes in a motor’s operating speed translate into a major change in the imposed load and annual energy consumption. Fan and pump “affinity” laws indicate that horsepower loading on motors by centrifugal loads varies as the third power or cube of its rotational speed. So a small increase in motor speed of 20 rpm can cause a 3.5% increase in electrical load.

5. Soft-start equipment on big electrical motors cuts utility demand charges. Soft-start equipment can lower your utility bills, but it will not significantly reduce demand charges. When motors start, they draw an “inrush” of current, often five to six times the motor’s full-load running current. This creates heat, a motor’s enemy. Soft starters increase the voltage applied to motor terminals over time, and this limits the inrush current and reduces heat buildup. In doing so, soft starters extend motor lifetimes, in particular, for motors frequently stopped and started.

Demand charges from utilities, however, are not affected. If electrical kilowatt demand is measured and billed on your utility account, the electric meter measures the average kW consumed over each 15 or 30-minute period. In contrast, soft starters affect motors’ power draw over the course of just a few seconds. The motor’s lower power draw over that short period is insignificant compared to the time period when demand charges are calculated.