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2:05 pm
December 1, 2004
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Clutch Assembly Aids in Condition Based Monitoring

The installation of a new clutch assembly using a torque monitoring device has saved a General Motors manufacturing facility in Spring Hill, TN, more than $1 million. The first prototype of the clutch was introduced in 2000 and savings are predicted to grow as more units are installed. Currently, equipment in the vehicle systems area of the plant is being upgraded to include the “smart” clutch.

The clutch is being used on drive units that power a vertical lift and transfer system in the assembly plant where the Saturn VUE small sport utility vehicle and the Saturn ION sedan and quad coupe are built. This assembly is part of a condition monitoring program that is being used to optimize maintenance at the facility.

Avoiding downtime
Significant downtime has been attributed to the vertical lifts and transfers which move the vehicles through the assembly process. These enormous drive units carry large amounts of weight which causes massive torque loads and sizable vibrations. These drives can fail for a variety of reasons: gearbox failures, motor failures, bearings, sprockets, etc.

If one of these drives fails, the line stops. Although most units are equipped with a backup drive, it can take maintenance personnel anywhere from 45 min to 1 hr to manually switch to the backup. A projected loss of up to $3500/min makes this downtime extremely expensive.

Prototype installed
A prototype clutch, designed jointly by Autogard Corp., Rockford, IL, and GM Spring Hill, was installed on selected lifts. The clutch quickly switches between drives and can be coupled with a torque ring and telemetry system for continuous monitoring of the torque and velocity of the drive shaft.

Using the torque ring outputs, the primary drive can be switched automatically to the secondary drive in 1 min rather than the 45 min at minimum that it takes for a manual changeover. Because this switch occurs automatically, the operations continue to run smoothly, allowing the maintenance personnel to redirect their work to other priority areas.

1204saturnfig1

Fig. 1. Right side angle shot with lifting carriage in full down position

In 2000, the first clutch and torque monitoring system was added to a vertical transfer that delivers the trunk lid from the upper to the lower chain conveyor for assembly. It has two identical drives but operates only one at a time. If one drive has a problem during production, employees quickly switch to the other drive by disassembling the flex steel coupling on one side and re-assembling the coupling on the other side (Fig. 1). Adding the smart clutch reduced the changeover time drastically and saved $157,000 every time a changeover was required during production.

Reduced interruptions in the flow of parts to the line and the stream of vehicles through the plant results in improved uptime and lower operating costs. The end result is that the maintenance department and operations management see improved efficiencies that favorably impact the bottom line.

Monitoring identifies problems
The data ring monitoring system provides key running load information on the condition of the vertical transfer. In one instance, information from the new unit identified a problem caused by a gearbox failure during the first 5 min of operation. The limit switches and spring preloads on the vertical transfer were adjusted incorrectly resulting in overtorque of the gearboxes by a factor of four times the maximum rating of the gearbox.

The monitoring system was used to aid in the proper adjustment of the limit switches and preload springs to bring the elevator back to its designed parameters. The system facilitates the detection of developing problems and allows the scheduling of maintenance based on conditions requiring attention before an impact to production occurs.

The result has been improved preventive maintenance scheduling, more efficient use of maintenance personnel, and early detection of mechanical problems through continual monitoring of the equipment.

Once the torque ring receiver is attached to a programmable logic controller, a computer program is then able to monitor the torque and velocity readings. If the threshold is reached, the system warns the maintenance team that something is about to fail. The monitor can be used to determine how much counterweight is needed.

Lessons learned
While considering the implementation of this project throughout the plant, several key lessons were learned:

  • If designed correctly, the clutch can be an extremely cost effective, efficient, and useful way to switch to the backup drives. In the current situation, it paid for itself the first time that it was used.
  • The torque monitor is not always required for brake solutions.
  • The torque monitor is not required for clutch implementations but without the data ring, the clutch cannot switch automatically between the primary and backup drive. With the torque monitor, the clutch switches to the backup during a failure, and provides an indication that the primary has failed. The data ring also can warn of imminent failure, allowing repair or replacement of the primary drive during scheduled downtime.
  • If a condition based monitoring system is in place, it is not imperative to have a backup. However, it should be evaluated on a case-by-case basis, depending on the criticality of the equipment and the efficiency of the maintenance organization. If planned downtime and occasional unplanned downtime can be tolerated, then the data ring should be sufficient.

Understanding each unique environment will help to ascertain which components and strategies need to be implemented. MT


Thomas A. Rogers, Ph.D, is vehicle systems engineer at General Motors’ General Motors Spring Hill Manufacturing, 100 Saturn Parkway , P.O. Box 1500, Spring Hill, TN 37174-1500. He can be reached at (931) 486-6782

How the System Works

The predictive part of the clutch and data ring assembly is the monitoring system. It is composed of a torque ring, telemetry receiver, and a torque monitor (Fig. 2).

1204saturnfig2

Fig. 2. The system is composed of a torque ring, telemetry receiver, and the torque monitor.

The torque ring is the heart of the system. It measures and transmits real-time torque data and can be installed virtually anywhere in the drivetrain. The torque ring consists of a battery power supply, strain-gauge bridge assembly, microprocessor-based system to interpret the strain-gauge data, and an electronic data transmitter. It is mounted in a 1-in.-thick aluminum or stainless steel ring. The data from the ring is transmitted as a 10-bit digital signal using an FM radio signal.

Angular and axial loads are isolated to ensure accurate torque measurements, and the torque can range up to 500,000 lb-ft. The ring cannot monitor high-frequency vibrations, but it can handle low-frequency signals less than 10 Hz. The torque ring is selected based on the drive location, torque requirements, and the drive train component to which it is to be adapted, such as a coupling, gear, or sprocket. The torque ring should be placed as close as possible to the part being analyzed.

The first step in installation is deciding which part of the machine/drive is to be analyzed or used as the basis of control. This position also must be the best position for measuring the torque directly rather than through a gear reduction. In some instances, by positioning the ring closer to the prime mover, the torque data can indicate changes in the drive train performance or gearbox efficiency. According to Autogard, this system is accurate to ±5 percent full scale.

The telemetry receiver picks up the radio signal containing the torque data transmitted from the torque ring aerial. It is enclosed in a small plastic box positioned approximately 5 mm from the aerial. A 6 m shielded cable connects the receiver to the torque monitor. The torque monitor is the control and display unit for the system. It is wired to the receiver, which provides the torque monitor with the true torque data from the equipment. Torque is displayed in the most relevant form for the environment. Trip points, relays, analog outputs, and an optional serial communications link for process or production control also are available. The trip points are extremely important in this sort of application.

The system allows for three programmable trip points, which can be set at different load levels to provide a variety of warning or control signals. The first trip is generally configured as an underload so that it will activate when the load falls below a set value. The second trip is typically configured as an overload so that it will activate when the load climbs above a set value. The third trip is typically configured as an overload and set above the second trip level to signal the end of a process/production cycle or to protect the equipment by shutting down the motor. The monitor can be set to hold this peak value, which can be useful when running the equipment again.


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