What You Should Know About Inching Drives
Although an inching drive may be used only a few times a year, it is at those times that it proves its real value. By permitting a bucket elevator or conveyor to be literally "inched" through its cycle, inspection and maintenance can be made more efficiently. The evaluation of maintenance needs, or spotting of malfunctions, are therefore greatly enhanced.
The common alternative to an inching drive – jogging a conveyor or elevator from position to position with the starter switch – is not generally practical or successful. The practice is lacking in accuracy, and could place damaging stresses on the system, especially with large motors and heavy loads. On some applications an inching drive might be considered mandatory in order to protect equipment from serious damage.
Field modifications can be made to retrofit an inching drive to an existing main drive, if factors are favorable. Also, portable inching drives are available to provide a practical and economical answer when a number of similar driven units are used.
Matching Two Drive Systems
Because an inching system must be closely matched to the main drive reducer, the two should be selected and designed as a complete package whenever possible. None of the elements are complex when taken separately, although the total can make up a fairly sophisticated package. Therefore, the interrelationships of the components must be carefully considered to assure proper operation and trouble-free service.
First, The Main Drive
When selecting the main drive for your elevator or conveyor, a number of factors must be considered. Number one is the service factor, which should be based on the duty cycle and nature of the application. The more demanding the service, the higher the service factor, with 1.0 serving as a base index. The manufacturer can assist you in arriving at a proper figure. Next, consider the configuration of the reducer to be used: parallel shaft, concentric shaft or right angle design. Selection will depend on space limitations, horsepower requirements, and design of the supporting structure.
For example, if the horsepower requirements are low (under 75 HP), the simplicity,
economy, and space savings of a concentric shaft speed reducer may be the answer.
However, one drawback to the concentric design is that if the reducer is in
line with the head shaft of the conveyor, then the motor will hang too great
a distance from the center line of the elevator. (See Figure 1) As a result,
the designer would be required to make the elevator structure heavier than would
normally be required.
Figure 1
The other drawback is that the input shaft on the reducer would have to be extended over the standard length to accommodate the high speed coupling and overrunning clutch with the chain and sprocket drive. This, in turn, would necessitate a longer than standard bedplate or motor bracket. The end result would be a "special" assembly, which would make it more difficult to obtain spare parts.
Overall, because the considerations are so many, it makes good sense to lean on the expertise of an established full-line manufacturer when selecting your main drive.
Next, High and Low Speed Connections
Once the main reducer configuration has been selected, and has been sized correctly for horsepower, speed, and class of service, then the high speed and low-speed shaft connections can be selected. If the low speed connection is a chain and sprocket arrangement, then the overhung load value of the low-speed reducer shaft must be considered to make sure it can adequately handle the stresses it will face.
Make sure the tight side of the chain is on top of the arrangement. Many installations have the slack side of the chain on top. When a drive is installed in this way, the chain will stretch and tend to wrap itself around the driving pinion, resulting in either a snapped chain, a broken sprocket, or a failed reducer.
If the drive must be installed with the slack side of the chain on the top, an idler sprocket should be used to prevent this problem from occurring.
Advantages of Fluid Couplings
The high-speed shaft connection will generally use either a flexible metallic element coupling, an elastomer element coupling, or a fluid coupling. In recent years the fluid coupling has gained favor in the United States for a number of reasons. First, if it is selected properly, a relatively small horsepower motor can be used to drive the system. That is because the fluid coupling is able to take advantage of the motor's pullout torque to start the system rather than its starting torque. It also brings the system up to speed at a slower rate, which puts less strain on all driven components of the system.
Matching the Inching Drive
Since an inching drive may be used as infrequently as one or two times a year, the service factor is usually Class 1 or 1.0, based on motor horsepower. Although the unit is normally designed to operate an empty elevator or conveyor, it should be capable of running the equipment fully loaded at a reduced speed in order to clear the belt or the buckets.
Elevator manufacturers will tell you that when an elevator is fully loaded, and run at an inching speed, the buckets will not discharge properly, causing the material to dump into the bin from which they are loading. While this is true, the size of the inching drive is so small in relation to the main drive that the cost difference for a drive capable of running a loaded elevator is small.
Most often, the reducer for the inching drive will be of concentric shaft design and probably will use no more than two or three horsepower to drive the system at the inching speed. This reducer would then be mounted piggyback with the modified motor mount for the main reducer, connected to the reducer high speed shaft (or in the case of a right angle unit, the intermediate shaft) with a chain and sprocket arrangement.
An overrunning clutch is mounted on the shaft of the main reducer. (See Figure
2) It is important that the inching drive motor not be so large that it will
over torque any of the other components of the system. Even though the torque
of a small horsepower motor may not seem significant, when you reduce the speed
to the main drive by a factor of 10 or 15, (depending on the combined ratio
of the chain and sprocket and inching reducer) the torque multiplication at
the low speed shaft of the reducer can be sufficient to damage the components
of the system.
Figure 2
Next, the overrunning clutch and chain need careful consideration. Remember that when the overrunning drag torque figure for the clutch is determined, it may be sufficient to cause the inching drive reducer and motor to run while the main drive is in operation.
If this happens, the high-speed shaft of the reducer and motor may turn at an undesirable 30,000 or 40,000 rpm, depending on the reducer and chain ratio of the drive package. To prevent this from happening, a small brake may be required for the inching drive motor. An interlock system should be used on the two motors so both may not be engaged at the same time.
The Lube System
Lubrication, of course, must also be considered.
When the main reducer is operating under average conditions, a normal splash lube system should be sufficient. But if the reducer is operated at low input speed for a period of time, this splash lube system may not be able to provide adequate lubrication to the bearings. In this event, depending on the design and configuration of the main drive unit, the lubrication system would have to be modified by adding oil dams for the bearings and the high speed gears, or an auxiliary lubrication pump and motor should be used. If this last system is used, it should be wired so that it operates with the startup of the inching drive motor.
All things considered, the proper selection of your inching drive and its related components will pay handsome dividends in more convenient servicing and repair of elevator and conveyor systems.
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