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Posted on Aug 12, 2020
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.
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.
A wide range of mechanical speed reduction devices includes pulleys, gears, pinions and friction drives. There are also electrical products that can change the speed of the engine. This discussion will focus on closed drive speed reducers, also called spur drives and gearboxes, which have two main configurations: in-line and perpendicular. Each of them can be achieved with different types of gears. Linear models typically consist of helical or spur gears, planetary gears, cycloidal mechanisms, or harmonic wave generators. Planetary designs generally provide the highest torque in the smallest package. Cycloidal and harmonic drives offer compact designs with higher ratios, while helical and helical gear units are generally the most economical. They are all quite efficient.
Worm gears are perhaps the most cost effective reduction solution, but they usually have a 5: 1 minimum and lose significant efficiency as gear ratios increase. Bevel gear reducers are very efficient but have an effective upper speed reduction limit of 6: 1. The type of application determines which speed reducer design best meets the requirements.
Before selecting any reducer, technical data must be gathered to properly select and install the unit: torque, speed, power, efficiency of the reducer, service factor, mounting position, connection variables and required service life. In some applications, the amount of play, transmission error, torsional stiffness, and moment of inertia are also important.
:: Read more : Why You Should Use Speed Reducers!
The required torque is perhaps the most important criterion as this translates into the amount of work the speed reducer has to do. While in simple applications, determining the torque can be relatively straightforward, it can be difficult in complex machinery. Inertia, friction, and gravity – the physical phenomena that tend to resist motion – have to be identified so that enough torque can be generated to overcome them. this can be difficult with complex machines. Inertia, friction, and gravity - physical phenomena that usually resist motion - must be identified in order to generate enough torque to overcome them. Taking into account the coefficients of friction and the acceleration and braking of inertial masses is important in calculating the required torque. The shortcut to finding the required torque for an existing machine is to read the motor current by determining the current draw. Then calculations can be made to find the required power. Finally, by using the standard torque formula and taking into account the different ratios, the final torque value can be obtained.
After determining the required power, the service factor must be taken into account to properly size the device. The service factor takes into account other operating parameters, including the length of working days, the number of starts and stops, load characteristics, and power sources. Most reducers are rated for a maximum torque at a given number of lifetime hours. The limiting factor in these ratings is not the gear or shaft strength, but the bearing life. Because bearings must support the separation forces of the gears under load, loading less than the maximum rating increases gearbox life. Conversely, by increasing the load variables as highlighted above, a decrease in gearbox life will result. Therefore, to arrive at the effective torque requirement, the appropriate service factors need to be applied.
Speed reducer and motor can be selected at this stage. Typically, a primary power source is selected, such as an engine or motor that runs at a certain speed. Obtaining the correct gear ratio of the speed reducer and the resulting multiplication of the torque is only a matter of dividing the motor speed by the speed of the driven element. Then the correct motor size can be found by attaching different factors and values to the standard motor power formula.
After the selection is made, the next question 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 controller and driven load.
Shaft orientation is one of the first considerations. In many applications, it is desirable to position the input or output shaft vertically. In this case, great care must be taken to ensure proper lubrication. Gearbox oil or grease not only protects against gear wear, but also reduces bearing wear. Thus, when one of the shafts is mounted vertically, the uppermost support bearing may not get the required lubrication. In some gear designs, splash and fogging from gears rotating in the oil reservoir are sufficient to ensure proper lubrication, but for slow-speed types, pre-lubricated, sealed bearings must be fitted. In still other high speed applications, it may be necessary to use internal or external pumps to deliver the lubricant to the desired location. Whenever it is necessary to mount the shaft vertically, it is important to determine if an alternate method of lubrication is required.
The next issue is how to connect the speed reducer to the power source and the driven load. Options include driving with a pulley, rack or pinion, linking to a clutch, line shaft, or universal joint, and shaft mounting directly on the driven shaft.
When connecting to a pulley, sprocket or gear, the main problem is the radial load, commonly known as the overhang load. Shaft bearings are designed not only to support the forces separating the gears, but also to transmit a certain radial and thrust load to the shafts themselves. When driving with pulleys and gears, there is a radial force as the belt or chain tries to turn the shaft. The magnitude of this force can be calculated as the transmitted torque divided by the radius of the pulley or sprocket. Usually, however, this is not the only side force exerted. The pulley or chain is tight on the drive side but has play on the rear side. A tensioning device is usually installed to reduce noise and prevent the belt from slipping or skipping teeth. When the belt or chain is tight, an additional radial load occurs. When selecting a gear drive, the combination of radial load resulting from torque and tension must be taken into account.
When connecting the speed reducer to the clutch and to a lesser extent to the line shafts and U joints, alignment is the main problem. Flexible couplings are recommended due to machining tolerances of gearbox housings and mounting plates. Without exact alignment, the use of a rigid coupling can place excessive side loads on the shaft bearings. Even with flexible couplings proper alignment is necessary as most couplings only allow 0.005 to 0.010 inch parallel misalignment and 1 to 3 ° angular misalignment. Many clutch designs are suitable for a variety of applications, but for maximum reducer life, the clutch should fit the task.
A third option for connecting the gearbox is to mount it directly on a driven shaft with a hollow bore output shaft. This reduces concerns about alignment and radial loads and saves space. A support arm from the gearbox to the machine frame prevents the gearbox from rotating around the shaft.
Multiple gear designs allow the motor to be directly mounted to the gear unit. These designs contain either extremely precise flanges to allow direct connection of the motor to the reducer or other adapters with integrated couplings. This eliminates the need to mount the motor separately, but this is usually only practical with smaller motors.
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