Torque overloads can cause serious problems for rotating machinery, but an understanding of torque limiting technologies can help engineers to enable more reliable operations
Reliability, safety and productivity are key terms that come to mind when considering manufacturing plant operations, especially those utilizing rotating machines. In the chemical process industries (CPI), there are difficult applications, such as those using continuous mixers and melt pumps, that operate with the risk of torque overloads. Overloads pose a threat to operational safety and can cause catastrophic machine failures. For example, failures caused by unexpected machine jams and electrical-grid-induced motor torque spikes can be severe enough to impact a chemical plant’s production for days, potentially stretching into months. A rotating machine whose drive-chain design does not fully account for torque spikes caused by sudden machine jams or electrical-supply faults will expose the weak link, either during the occurrence of the torque spike or sometime later when the fatigued link finally breaks unexpectedly.
There are several ways to protect a drive chain. One method is to purposefully design the drive chain with a weak link, such as a closely sized flexible-connection coupling. Some level of perceived protection may be achieved through this method, but it must be considered that a failed flexible-connection coupling will require some degree of parts replacement and could cause damage to the surrounding drive components and guards, as well as requiring added downtime, parts and labor. The idea of a weak link in the drive chain is a good one, but that weak link needs to provide torque accuracy to maximize machine productivity, be easy to reset and not require a lot of parts or manpower. One approach to address the problem of maximizing machine productivity without exceeding machine capacity is the use of a torque limiter. There are many types of equipment within a CPI facility where increased reliability, safety and productivity can be realized from the application of torque limiting technologies. These include steam and gas turbine drives, compressors, expanders, centrifuges, fans, reactors, crushers, mixers, mills and pumps.
Torque limiting technologies
Torque limiters are commonly used in stationary and mobile applications. They can be applied to equipment requiring less than 1,000 Nm and more than 15,000,000 Nm of torque, and operate at speeds from 1 to 10,000 rpm or more. Some limiter configurations are summarized in the following sections.
Shear pin couplings. Shear pin couplings are fatigue-based limiters that use specifically sized pins to transmit a set amount of torque and require replacement after an overload or after they have experienced too much fatigue. Adjustment of the torque limiter capacity requires resizing of the shear pins.
Ball-in detent couplings. These are spring-loaded limiters that can be reset through counter-rotation of the mounted parts or by re-engagement of the balls via taps of a mallet. Over a period of time, the detents wear and the spring’s tension decreases. Adjustment of the torque limiter capacity can typically be done through adjustment of the springs that apply pressure to the balls.
Pneumatically pressurized, dry-running friction clutches. This type of clutch uses a clutch pack, which is a stack of clutch discs and steel discs. The clutch is engaged using an air bladder that is fed plant air, either axially through a drilled machine shaft or radially via a non-rotating portion of the friction clutch assembly. The torque-transmission capability of the clutch can be regulated by the applied air pressure. However, the torque-transmission capacity will decrease with any air leaks that occur, causing the clutch to slip and potentially overheat during normal machine operation. To avoid overheating the clutch in any operating scenario, the clutch must be monitored for extended periods of slip in order to avoid overheating and the potential for clutch failure.
Hydraulically pressurized, friction-based couplings. These couplings (Figure 1) are backlash-free torque limiters that utilize shear tubes and a shear ring for release of the hydraulic pressure during a torque overload event. As soon as the pressure is released, the coupling freely spins without contacting the friction surfaces. The shear tubes are replaced and hydraulic pressure is re-applied based on the desired torque limitation to re-engage the unit. Adjustment of the torque limiter capacity can be accomplished by referencing a pressure-versus-torque calibration diagram that is typically supplied with a unit.
Mounting configurations of the previously mentioned torque-limiter types include, but are not limited to, a drop-out spacer design, a shaft-mount design that would replace the solid hub of a connection coupling and a design for integration with the flexible hub of a connection coupling. Some designs allow for integration within gearboxes or machine components.
Torque limiter operation and setup
Having addressed the issue of safely maintaining machine reliability and production without exceeding design capacity, the next issue often occurs after the release of the torque limiter. It is true that the torque limiter has done its job to protect the drive chain of the machine by releasing, but it does not automatically reset to allow for a machine start. The requirement to physically reset the torque limiter is not necessarily bad, since it forces the operator to try to identify and understand the cause of the torque event that caused the release of the torque limiter. However, in some cases, the cause of the torque event may be readily known through instrumentation feedback in the control room. The torque limiter will still need some field attention to allow the machine to resume operation. This leads to the first question — is there a way to limit torque but not release? A second question would follow — is there a way to limit torque and release only in situations of long-duration torque overloads?
The answer to those questions will depend on the application and the characteristics of the overload. A potential solution is offered by the ability to slip instead of fully release during short-duration overloads. Rather than immediately releasing on overload, some torque limiters can either slip for a number of degrees before releasing, or never release at all unless an external trigger is used or the machine-monitoring controls force a shutdown. These torque limiters are based on the hydraulically pressurized, friction-based design, but use specially selected friction surfaces to allow for extended periods of slip. To help understand these limiters, it is important to know more about their construction.
A basic torque-limiting coupling of this type is made up of an inner and outer sleeve that are assembled and welded at the ends. This assembly forms a twin-walled hollow sleeve that can be oil-pressurized after the machining of the necessary pressurization and shear-tube ports have been completed. The design of the shear tube and mating seat provides a sealed system, while the size of the torque-limiting coupling determines the size and quantity of shear tubes that are to be used. The friction surface is treated to prevent wear during the slip phase of the coupling release. Once the coupling has released, it rotates on bearings, preventing wear on the torque-transmitting friction surfaces.
During normal operation, the bearings of the torque limiter remain static. The bearings only rotate following a release due to torque overload, which makes bearing life a minor factor when considering the operational dependability of the coupling. The bearings and friction surfaces are separated from the pressurized sleeve and require lubrication oil. The lubrication oil is used for two things: bearing lubrication during a release condition, and to maintain a predictable friction coefficient across the friction surfaces, which results in a precise-release torque relative to the applied pressure.
As noted, hydraulically pressurized, friction-based torque limiting couplings have no backlash and are not subject to material fatigue because the torque is transmitted through a friction surface. The applied hydraulic pressure generates a defined frictional force between the pressure sleeve and the shaft. The applied pressure determines the release torque of the coupling. Therefore, an increase or decrease of applied pressure, working within the torque limiters’ adjustment range, will result in an increase or decrease of the release torque.
If the operating torque exceeds the pressure-based-release torque setting, the driving shaft will rotate relative to the pressure sleeve that is connected to the driven load. This results in an immediate reduction in applied torque when the friction force changes state from static to dynamic. The shear ring that is fixed to the driving shaft rotates relative to the pressure sleeve and breaks off the top of the shear tubes. Upon contact, the oil pressure in the coupling drops, and the applied frictional force in the coupling is reduced, releasing the torque limiting coupling and providing full separation of the driving and driven components of the drive chain.
Following a release, the coupling must be reset. First, the shear ring is aligned to allow removal of the shear tubes. Next, the shear tubes are replaced and torqued to specification. Finally, the coupling is re-pressurized according to the calibration curve of the unit.
Advances in torque limiters
With the basics of torque-limiting couplings in mind, it is also important to understand the more advanced versions of these technologies, including: slipping torque-limiting couplings that have the ability to slip and eventually release to prevent damage from excessive slip due to machine overloads; and permanently slipping torque-limiting couplings that can only be influenced through external monitoring.
A hydraulically pressurized, friction-based, slipping torque limiter (Figure 2) is a mechanical-fault ride-through coupling (meaning it stays connected during periods of instability) that slips to trim or shave off torque peaks caused by short-duration overloads to protect the drive-chain components. This type of torque limiter was originally designed as a startup coupling for synchronous motor-driven centrifugal pumps and compressors. Synchronous motors generate high-amplitude, short-duration transient torques during acceleration and prior to grid synchronization. The slipping torque-limiting coupling can also fully release in situations of extended overtorques to protect the links of the drive chain, as well as the coupling itself. The technology and construction are based on the previously discussed design of the basic hydraulically pressurized, friction-based torque limiting coupling. However, there are differences that set the slipping torque-limiting coupling apart, as follows:
- It has the ability to slip in short durations without seizing due to its friction surface
- The slipping limiter device is centrifugally engaged by the shaft rotation of the application
- The slipping limiter design allows for a minimum slip before release of 30 deg and a maximum of 120 deg per start
The slip angle before release can be reset with each shutdown of the machine. This means that multiple short-duration torque events that cause the torque limiter to slip can be reset with a simple stop and restart of the machine, thereby providing the full 30- to 120-deg slip angle once more. With this type of torque limiter, the need for a machine shutdown is limited to instances of sustained short-duration overloads or a continuous overload. After a trip, the slipping torque limiter must be reset and re-pressurized for the appropriate torque. Furthermore, trips can be limited or avoided altogether with the use of an active slip monitor. The monitoring system can be used to provide feedback to the machine’s programmable logic controller (PLC) to either stop the process or reduce the load before the torque-limiting coupling is mechanically forced to trip (Figure 3).
Another type of slipping torque-limiting coupling provides mechanical-fault ride-through capability to slip and trim or shave off torque peaks caused by short- or extended-duration overloads to protect the drive-chain components. This type of slip-enabled coupling (Figure 4) will not release. Therefore, no resetting of the coupling is required. Once more, active monitoring of the slip is important to provide a window into what is happening with the drive chain. While the coupling is designed to slip, it is not designed for permanent slip. Slip creates heat and is an indicator that the machine is being pushed past its design parameters. The monitor can “look” at the torque-limiter slip and can provide the machine PLC with feedback to make appropriate decisions, such as feed reduction, feed stop or machine shutdown.
Reliability, safety and production demands on chemical plant operators and machines continue to increase as companies focus intently on operating efficiency and controlled costs. As outlined, these demands require a hard look at machinery to consider what can be done to maximize capacities without sacrificing the ability to operate safely and reliably. Torque limiting technology is designed to help maximize a machine’s productivity without exceeding design capacity or jeopardizing safety and reliability. ■
Edited by Mary Page Bailey
Todd Lehman is a product sales manager for Voith Turbo Inc. (25 Winship Rd., York, PA 17406; Email: firstname.lastname@example.org) specializing in the company’s SafeSet, SmartSet and SlipSet torque-limiting couplings. Lehman earned a degree in electrical engineering technology from Penn State, and he has been involved in the selection and application of mechanical power-transmission solutions for more than 25 years. In addition to torque limiting technologies, Lehman has worked with various mechanical power-transmission solutions at Voith Turbo, including hydrodynamic fluid couplings, universal joint shafts and diaphragm couplings.
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