Speed Reducers
Speed reducers are mechanical devices generally used for two purposes. The primary use is to multiply the amount of torque generated by an input power source to increase the amount of usable work. They also reduce the input power source speed to achieve desired output speeds.
Gear technology, types of gears and design differences
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.
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: The image above shows two different configurations for bevel gears: straight and spiral 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 degrees. 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 degrees, when mounted on perpendicular shafts. It's normal gear ratio range is 3:2 to 10:1.
Bevel Gears: 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.
The cross-section of the motor in the image above demonstrates how spiral bevel gears are used.
This engine is using a conjunction of hypoid gears and spiral bevel gears to operate the motor.
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 4:1.6.
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. The pinion and gear are often always opposite hand and the spiral angle of the pinion is usually larger than 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.
In the hypoid gear above, the larger gear is called the crown while the small gear is called the pinion.
Worm Gears: The model cross-section shows a typical placement and use of a worm gear. Worm gears have an inherent safety mechanism built-in to its design since they cannot function in the reverse direction. 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.
Integrating speed reducers
The selection and integration of speed reducers entails much more than simply picking one out of a catalog. In most cases the maximum torque, speeds, and radial loads
published cannot be used simultaneously. Proper service factors must be applied to accommodate a wide range of dynamic applications. And, once the appropriate speed reducer is
selected, proper installation and maintenance are the keys to maximizing life.
After selection, the next issue is how the gearbox will be integrated into the machine. The main concerns are how the gearbox will be mounted and how it will be connected to the
driver and driven load.
Shaft orientation is one of the first considerations. In many applications it is desirable to have either the input or output shaft vertically oriented. In this situation great
care must be taken to assure proper lubrication. The oil or grease in a gearbox does not only provide protection against gear wear, but also for reducing bearing wear. So, when
one of the shafts is mounted vertically, the uppermost support bearing may not get the lubrication it needs. In certain gearbox designs the splash and misting created by the
gears rotating through the oil reservoir are enough to provide the proper lubrication, but in low-speed types it is necessary to install pre-greased, sealed bearings. In still
other high-speed applications it may be necessary to provide internal or external pumps to deliver the lubrication to the appropriate location. Whenever a shaft needs to be
vertically mounted, it is important to determine if an alternate lubrication method is necessary. The next consideration is how the speed reducer will be connected to the power source and to the driven load. Options include driving with a pulley, sprocket, or gear,
connecting with a coupling, line shaft, or universal joint, and shaft mounting directly on the driven shaft.
When connecting with a pulley, sprocket, or gear, the main issue is radial load, commonly known as overhang load. Shaft bearings are not only designed to support gear separation
forces but to accommodate a certain amount of radial and thrust loading on the shafts themselves. When driving with pulleys and sprockets, a radial force occurs as the belt or
chain tries to rotate the shaft. The magnitude of this force can be calculated as the torque transmitted divided by the radius of the pulley or sprocket. This usually isn’t the
only side force exerted, however. The pulley or chain is tight on the driving side, but has some slack on the back side. To reduce the resulting noise and prevent belt slip or
tooth jumps, it is common to install a tension device. When the belt or chain is tightened, additional radial loading occurs. The combination of radial load due to torque and
tension must be considered when selecting a gear drive.
When connecting a speed reducer with a coupling, and to a lesser extent, line shafts and u-joints, alignment is the main concern. Because of machining tolerances on gearbox
housings and mounting plates, flexible couplings are recommended. Without exact alignment, using a rigid coupling could create excessive side loads on the shaft bearings. Even
with flexible couplings proper alignment is necessary, as most couplings will only allow parallel misalignments of 0.005 to 0.010 in. and angular misalignments of 1 to 3°. Many
coupling designs are appropriate for different applications, but for maximum reducer life, the coupling should suit the job.
The third option for connecting the gearbox is to mount it directly on the driven shaft with a hollow bore output shaft. This reduces the concerns regarding alignment and radial
loads and conserves space. A support arm from the gearbox to the machine frame keeps the gearbox from rotating about the shaft.
Many gearbox designs allow for the motor to be directly mounted to the reducer. These designs incorporate either extremely precise flanges to allow the motor to be directly plugged into the reducer or other adapters with integral couplings. This eliminates the need for separate mounting of the motor, but is usually only practical with smaller motors.
Although this completes the major considerations of speed reducer selection and integration into the machine, other elements are important in certain applications. For instance, with reversing or intermittent load applications, the amount of backlash should be reduced. The second element that should be considered is transmission error, or the positional variance of output motion relative to input motion. This is usually a function of gear and assembly quality and is important when precise and predictable motion is required. The third design element is torsional rigidity, which is a reducer’s resistance to twist under load. This is an especially important consideration when precise motion must be maintained during acceleration and deceleration. The final design element is moment of inertia. In fast acceleration applications, such as servo systems, the gearbox inertia increases the motor torque required to move the load. All of these speed reducer elements can be supplied in varying levels of precision or durability with increasing cost for more stringent requirement.
Lubrication...
All lubricants minimize friction and result in lower heat generation. However, oil provides the best lubricating properties for gear motors and is typically used in 1/10 horsepower and
larger gear motors designed for industrial service. Increasing the service life (over 10,000 hours) can be accomplished with a circulating fluid lubrication system. The fundamental
characteristic of oil is its free flow and constant presence at the tooth surfaces of a gearhead during operation. The oil needs to provide a consistent and continuous
lubricating film at the load zone, while withstanding dynamic forces such as centrifugal forces or severe loading that can prevent the lubricant from doing its job.
Lubricants used in parallel shaft
gear motors (which usually employ spur or helical gearing) are relatively less critical than those for right angle worm-gear types. Usually,
mineral-based oils with EP additives suffice if the proper level is maintained. Some fractional horsepower gear motors use hydraulic-type oils to decrease gear shaft or journal wear.
Right-angle gear motors with worm or other sliding-contact gearing require careful attention because the lubricants reach higher operating temperatures due to lower inherent
efficiency. Such lubricants generally have higher viscosity and contain protective additives to prevent oxidation and to enhance “oiliness.”
Despite its advantages, oil is not commonly used in smaller
gear motors because of sealing issues. Smaller gear motors usually do not have large gasket surfaces and may not have
sufficient power to withstand the increased friction of a contact seal on the rotor shaft. Therefore, grease is used as a compromise in most gear motors under 1/4 horsepower. Compared
with oil, grease provides less consistent lubrication to the gear teeth under load and gear life can be reduced by up to 50%. However, grease provides mounting flexibility,
minimizes leakage risk, and eliminates visual oil level inspections.
With regard to operating conditions, lubricant life in gear motors is directly related to temperature. Generally, within normal operating ranges, lubricant life doubles for every
25° F decrease in temperature. Gear motors operating in high or low ambient temperature ranges require special lubricants such as synthetic lubricants, or a lubricating system.
Gaskets, motor insulation, and lubricant life may be seriously affected by temperature extremes. When other than normal ambient temperatures (32° to 104° F) are expected,
consult the motor manufacturer.
Although the proper Sizing, Profile Definition and Load Characteristics are important to proper selection and full life of the product, these requirements will be addressed with you by our application representatives.
There are a number of types, sizes and designs for you to consider in any NEW project. And if you're replacing your present drive system, you may want to consider an upgrade to a different design because of efficiency.
The links below will take you to a more detailed description of some of these numerous designs.
Call A.R.&E. for your next speed reducer application or project.
Right Angle Speed Reducer Description: Common in industrial and commercial use. Reasonably inexpensive and significant ratios available (5:1 to 1000:1). Con... inefficient. High ratios will cost you dearly in excessive heat and power losses. |
In-Line Speed Reducer Description: Flat belts and sheaves are used for Power Transmission but have been around for ages. A major use is as a conveyor line, moving products in warehouses and outdoors like a quarry. |
Parallel Shaft Speed Reducer Description: Synchronous and Timing belt systems are used primarily when the driven equipment MUST remain in total synchronization, such that the speed of the driver and driven shafts must experience ZERO slip. |
Bevel Gear Speed Reducer Description: This is a "Mechanical" type of Variable Speed system, as compared to today's electronic drives. But even these mechanical systems can be somewhat automated. Rather than having to "turn a crank" to modify the speed, some systems have that portion driven by a small motor. |
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