Bulk Engineering, Technical articles

Rail clamp failure modes and maintenance

Rail clamp inspections carried out by Aspec Engineering have revealed that, on average, over half of the units inspected are not working properly. Paul Munzenberger reviews the types of failures that have been found and the routine inspections needed.

Modern mobile material handling equipment such as stackers, reclaimers and ship loaders use rail clamps similar to the ones shown in Figure 1 on their long travel motion to provide additional holding capacity to the long travel service brakes. 

Rail clamps are typically used to allow the machine to maintain its long travel position under the action of a wind speed greater than the maximum operating speed but less than the ultimate design storm wind speed. 

Recent experience has highlighted that wind speeds can rise from safe operating speeds to near full storm wind speeds within minutes in severe thunderstorm events. This is compared to the 30 minutes or more that it would take for a machine to travel to and be secured in a dedicated storm park location and to physically restrain the machine with pins or the like. As a result of several very serious incidents involving rail mounted machines, the role of rail clamps has changed to restraining the machine in much higher wind speeds or where possible under full wind conditions for an event where it is not possible to move the machine to the dedicated storm park.

Normally, there will be one or more rail clamps on each long travel rail. As the name suggests, rail clamps function by gripping the rail between clamping jaws that typically have a serrated pattern cut into the gripping face. Similar to the long travel high-speed motor brakes, rail clamps are fail-safe as the clamping force is created by a spring pack that acts on a linkage with a large mechanical advantage. The clamping force is typically released by a hydraulic cylinder that compresses the springs to release the clamps.

Two different categories of rail clamp are available: the first, and most common, grips the sides of the railhead and the second type features a single serrated platen that is pushed onto the top of the railhead using the mass of the machine to generate the clamping force. The second type of clamp is used where some feature of the rail prevents the side clamping type from being used — a rail embedded in concrete for example. The push down type is less effective as they are limited by the mass available to provide the clamping force, and they reduce the braking effectiveness of the braked wheels because they remove mass from them.

Figure 2: A schematic of a rail clamp.
Figure 2: A schematic of a rail clamp.

Although the rail clamps assist the service brakes, their operation is very different. While a machine is operating, the rail clamps will typically stay released for periods of 10 to 15 minutes and every time a long travel movement is executed, the rail clamp timer will be reset. This means that for a period of operation, the rail clamps will not be applied until after the machine has completed its task. 

For an emergency stop, the application of the rail clamps will be slowed by their hydraulic system to allow the long travel service brakes to bring the machine to a halt within eight to 10 seconds before the clamp jaws touch the rail. This is because, unlike service brakes which are designed to be applied on a moving machine, the rail clamp jaws will be damaged if they grip a rail while the machine is still moving.

Typical rail clamp failures

Over many mechanical inspections conducted by Aspec Engineering, it has been found that well over half, and likely as much as three quarters, of all rail clamps have not been functioning correctly. Sites have been inspected where the entire fleet of rail clamps is not working properly.

Unfortunately, when a rail clamp is not working correctly, it most often means that the clamp will afford zero additional restraint to a machine and leave the machine vulnerable to a potential accident.

The most common failure type for rail clamps is that the jaw serrations have worn out, resulting in reduced clamping effort or no clamping effort at all. Worn jaws like those shown in Figure 3 can be a result of neglect, or the clamps being regularly applied while they are still moving.

Figure 3: A view of a rail clamp jaw that has minimal contact with its rail.
Figure 3: A view of a rail clamp jaw that has minimal contact with its rail.

Poor rail alignment or wear in clamp guide wheels can also result in the jaws rubbing along the rail which wears away the serrations. Poor condition of the rail can also result in reduced clamp effectiveness. The rail can be dirty or greasy which reduces friction and the available clamping resistance. The top edges of the rail head can be rolled over (mushroomed) by repeated passing of the machine which causes burs to form on the edge of the rail head. When the clamp grips the burr, the engagement of the jaw is poor and there is a risk that the burr will fail in shear. It is also likely that the clamp will slip on the burr which will wear a grove in the jaws that reduces the clamping area and increases wear rates on the remaining jaw surfaces.

Improper grinding of the rail, to remove the metal flow, can cause oversized radii on the corners of the rail head. Large radii can mimic the effects of a rail clamp that is installed too high above the rail where reduced clamping area becomes a problem and, in extreme cases, the apparent misalignment can be bad enough that the rail clamp can pop up off the rail and not clamp at all. Misaligned rail, through subsidence or other mechanisms, can also result in the rail clamp forcing itself off the rail.


The number of defective rail clamps that have been found over years of inspections indicates that they are not typically inspected as part of routine inspection or that the inspectors are not familiar enough with mechanical equipment to recognise when a rail clamp is defective.

Figure 4: A rail clamp mechanism that has reached the end of its travel.
Figure 4: A rail clamp mechanism that has reached the end of its travel.

Regular rail clamp checks should be carried out as part of routine maintenance. Weekly or monthly checks should include the following items:

  • Make sure that the exterior and interior of the unit are clean and free of debris that can prevent the mechanism from operating.
  • Ensure that the hydraulic unit and actuating cylinder are free from leaks as lubricant on the jaws and the rail will reduce the clamp’s effectiveness. Stray lubricant from the long travel wheels can also make its way onto the rails and reduce the clamp’s effectiveness as well. While checking for leaks, the rail clamp’s mechanism should be lubricated if required.
  • Regular inspections should be carried out while the clamp is applied so that the resting configuration of the clamp can be checked for overtravel either through a mechanical indicator or by determining if the application mechanism has reached the end of its travel as shown in Figure 4. A clamp mechanism that is travelling beyond its normal travel will have a reduced clamping effect due to the lower spring force at longer travels and possibly an additional loss of effectiveness due to the loss of mechanical advantage in the mechanism.
  • Actuate the clamps locally to make sure that the rail clamp mechanism is moving freely and that the pressure of the hydraulic system is within its operating specifications.
  • Check the released position of the mechanism while it is being actuated.
  • The emergency stop performance of the clamp should also be checked to ensure that that the clamp takes the required amount of time to apply.
  • The release pressure of the rail clamp should be checked with a pump off test where the hydraulic pressure to lift the rail clamps is measured. The pressure required to release the rail clamps is an indicator of the clamping force and will also provide an idea of the condition of the rail clamps.

The condition of the clamp jaws should be checked every six months. This is the most critical step that is commonly not carried out. The check involves clamping cardboard or brass shim between the clamping faces and the rail to imprint the pattern of the serrations onto them as shown in Figure 5. The serration pattern thus developed will give an indication of the state of the serrations and whether the clamps are gripping the rail correctly. 

Figure 5: Four rail clamp platen test papers.
Figure 5: Four rail clamp platen test papers.

The use of cardboard or brass will depend on the clamp manufacturer as will the analysis of the imprinted pattern to determine if the platens are to be replaced or not. This check should be carried out away from the storm park position on rail that is well used, and at multiple locations to properly check the condition of the rail and alignment of the clamp.


Rail clamps are an important component of rail mounted material handling machine restraint especially with the increasingly common requirement that the rail clamps, working with the service brakes, can restrain a machine in higher winds even up to a full storm wind event without the need to move to a storm park location. Not only is this increased requirement being used in the design criteria of new machines, but existing machines are also being retrofitted with rail clamps and machines that already have rail clamps are being fitted with larger units to provide additional restraint. 

Where it is available, the increased function of storm restraint capable rail clamps is being incorporated into site operations and they must be expected to work when their assistance is required. It is critical that the rail clamp manufacturers maintenance instructions are adhered to, that rail clamp maintainers are well trained to carry out the appropriate checks and that their findings are acted upon to ensure continued reliable operation of rail clamps and safety of the machine.  

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