The mounting requirements of a hydraulic swivel are a tad bit more important than a casual observer would ascertain from a brief observation. Mounting rigidity and swivel torque are important design considerations. In this blog, I will discuss torque and mounting rigidity and how they interact with each other.
The rotational torque of the hydraulic swivel is what is required to overcome the resistance to turning. There are a few general concepts that one can use to judge the optimal hydraulic swivel rotational torque:
- There is a “breakaway” torque value when first starting the movement from a static position
- After the rotation has begun, the dynamic rotation value should be lower
- The friction between the seal and sealing surface creates resistance to turning
- The larger the seal diameter, the higher the resistance to turning
- The more seals, the higher the resistance to turning
- The higher the psi within the circuit, the higher the resistance to turning
- Seal geometry affects resistance to turning
- Seal material affects resistance to turning
- Non-pressurized ports create lowest resistance to turning
- Every other port pressurized creates highest resistance to turning
- All ports pressurized equally creates similar torque to non-pressurized configuration
- Different permutations of pressured ports will create a wide variety of swivel torques
Steady state rotational torque isn't the only concern when dealing with mounting requirements of a hydraulic swivel. The seals within the swivel have static and dynamic friction. The static friction is higher than dynamic friction. Just like pushing a box across the floor, it is harder to start it moving than it is to keep it moving. The static resistance to rotation in a hydraulic swivel will be much higher than the dynamic resistance to rotation.
The difference between static and dynamic friction is of great importance here. To better explain why this is very important I will describe a situation that can lead to the dreaded torque arm restraint chatter. The following would be a step-by-step description of how chatter develops within an assembly that contains a hydraulic swivel. I refer to the 'torque constraint assembly' as the method (typically a weldment) for driving the hydraulic swivel.
- Rotating member begins to rotate based on operator inputs
- Torque constraint assembly engages swivel and begins to put rotational force on swivel
- High static friction within swivel creates large resistance to turning
- Torque constraint assembly flexes slightly due to large resistance turning that swivel is creating
- As force continues to increase, the swivel transitions from static to dynamic friction and begins to rotate. It actually snaps forward farther ahead due to the torque constraint assembly acting like a sling shot.
- With the swivel farther ahead than the torque constraint assembly, it pauses for a brief moment
- Torque constraint catches up and engages swivel
- Cycle begins again at step number 2
The whole process happens many times per second (100Hz potentially) depending on the natural frequency of the torque constraint assembly. The natural frequency of the torque constraint assembly is usually within the audible range of humans, so when it is energized (steps 2-8), it makes a large noise that can be heard and felt.
I need to make a couple points that will provide the optimal mounting and torque restraint features in order to reduce the risk of several potential failure modes:
- It is not the swivel that is chattering, it is the torque constraint assembly and mounting of the swivel that is the source of the chattering issue
- This is NOT similar to a hydraulic cylinder that chatters in its linear application
- When a hydraulic cylinder chatters, it is due to resistance to stroke and a 'spongy' fluid power circuit (this is an entirely different discussion)
- The best ways to eliminate or prevent hydraulic swivel chatter and other mounting concerns are:
- Increase robustness into the stationary solid mounting plate and mounting lugs
- Increase torque arm constraint assembly rigidity
- If a design requires a parallel to the axial center line of rotation, the length of a torque arm should be as short and robust as possible
- Regardless of which end of the swivel faces up or down, choose first to orient your fixed stationary mounting and your rotating element close torque arm as close together as possible and stay as close to the center of the radial plane of the slewing ring as possible.
It would be difficult for UEA to provide design guidelines as there is a huge variety of different types of applications for hydraulic swivels. If you involve UEA designers early in the design process, we can provide additional design analysis and feedback to assist in creating a rigid mounting and torque constraint assembly. Torque constraint assembly flexation and natural frequency analysis can also be provided. Upfront teamwork and analysis will ensure your assembly and our hydraulic swivel will live together for many, many years!