Edited by: Mike Santora, Associate Editor
You’ve already accounted for potential misalignments, looked into thermal expansion scenarios and, well, checked to see what coupling was used the last time. So what’s next? Here, we cover some higher-level questions to address when designing the coupling element of your motion system equation.
1. What role does resonant frequency play in selecting a bellows coupling?
Resonant or natural frequency is the innate oscillation frequency of a system. It varies based on the geometry, arrangement, inertia and other factors in an assembly. Bellows couplings are meant to be torsionally stiff. That is, they are flexible in movements that are perpendicular or transverse to the shafts of the two coupled devices. As such, the natural rotational frequency of the coupling is the main concern. The coupling is designed to flex in those other directions, but it should transmit torque with as little vibration or oscillation as possible.
Resonant frequency, or natural frequency, is a concern when selecting bellows couplings. If not taken into account, an otherwise sound choice for a bellows coupling may perform poorly. Resonant frequency can be a serious destructive force if left unchecked. It may cause significant faults or outright failure.
We should be selecting bellows couplings that have a natural frequency that is far higher or lower than the motor’s oscillation frequency. We take this into account because the bellows coupling becomes a critical component in determining the natural frequency of the whole system. It is, after all, the bellows coupling that is transmitting the rotational motion. Its torsional stiffness is also an important factor to consider.
When selecting a bellows coupling, compare its resonant frequency to the speeds at which the system operates. Make sure that the speed of the system and the resonant frequency rating of the bellows coupling are sufficiently far apart from each other. Manufacturers often provide equations and recommendations for bellows couplings and their resonant frequency.
2. Why are tightening torques and fitting tolerances important for couplings?
Couplings and bearings must mount to components in a system. Because of this, they come with tolerances and assembly concerns. Tolerance refers to manufacturing tolerances and tolerances in installation and mounting. Tightening torque refers to couplings that screw onto shafts or other components in a system.
Tightening torque is the torque required to mount a coupling. If this is not done correctly, the bearing or coupling will not function properly. An overly-torqued, over-tightened bearing can cause over-stress in the bearing and cause it to fail prematurely. If it is under-torqued or under-tightened, the bearing may slip or “play” too much between the two components that are coupled together. This can cause inefficient operation or vibration. At worst, under-tightening can cause catastrophic system failure.
When mounting bearings and couplings, tightening torque should always follow the manufacturer’s specifications. Trying to gauge if the assembly is tight enough by “feel” or “intuition” is a dangerous practice and engineers should avoid such assembly techniques. If the information regarding tightening torques cannot be found in documentation provided with a coupling or bearing, ask the manufacturer.
Fitting tolerances refer to the variation between the actual size of the shaft and the rated values the coupling can accommodate. For example, if a coupling mounts to a 5 ± 0.1 mm shaft, then the tolerance is 0.1 mm. Usually, couplings that flex and bend have larger tolerances, as they can accommodate greater distortions. If using a coupling, make sure to check if the range in which movement occurs falls into these tolerances. Also make sure to measure the shaft with a degree of precision comparable to the tolerance. Attempting to use a coupling beyond its tolerances can cause it to fail or not be useful at all in a system.
3. How do you select a coupling when you expect heavy and percussive loads?
When selecting couplings for heavy and percussive loads, there are a few design considerations to keep in mind. These include the type of coupling, the operating conditions and the spacing of the couplings and shafts.
For the coupling hubs, the end that carries the load or otherwise interacts with the system is of primary concern. In heavy and percussive loading situations, make sure that this end is rigid enough to stand up to the rigors of operation. If, however, the system has requirements for flexibility or movement, a bearing that is rigid in one direction but flexible in another, such as a bellows coupling, may be more suitable. Safety couplings, or overload couplings can also be used in these types of applications. The safety coupling protects the overall system in overload or crash situations.
When it comes to ball and roller bearings, roller bearings can support heavier loads than a comparable ball bearing. This means that ball bearings are usually found in light loading applications, while roller bearings find use in more demanding situations. Bearings that use cages usually support less than those that have a full set of rollers.
Keep in mind what direction the load is coming from. Thrust ball bearings only accommodate for axial loads in one direction. Angular contact thrust ball bearings are good for normal axial loads that occur at high speeds. Heavier loads that are purely axial allow the use of needle roller thrust bearings, cylindrical bearings and tapered roller bearings. Spherical roller bearings are for axial loads in one direction only, as well as radial loads. For heavy, alternating loads, try using two cylindrical rollers or two spherical rollers mounted together.
Heavy, combined loads require unique combinations and types of bearings. These situations are often specialized; consulting with bearing manufacturers is helpful. Keep in mind additional support bearings that only handle certain kinds of loads, such as axial or radial, may also be required.
Also remember that heat, moisture and lubrication affect how well bearings and couplings accommodate loads. Reading the documentation provided by manufacturers is vital to make sure that the system is not placing bearings and couplings in unacceptable operating conditions.
4. How do you calculate the angle of twist on a line shaft?
The angle of twist on a line shaft involves its geometry, and the torque applied to it. This calculation determines the rotational displacement of a shaft given a certain load. The angle of twist calculations also involve determining the second area moment of inertia of a shaft. Engineers usually find these in a table of equations given a shaft’s shape and whether it is hollow or solid. Many sources provide these tables, including design guidebooks and manufacturers.
Here is an example equation to illustrate the concept. Say a shaft is solid and has a 70-mm diameter and is 2.0 m long. It is subjected to a torque from the system of 1500 Nm.
To solve, consider the following values. The diameter, D = 0.05 m, the shaft’s length, L = 2.0 m and the torque, T = 1500 nM. Additionally, J, the second area moment, needs to be calculated using equations found in documentation or standard charts. Beyond this, the calculations need the modulus of rigidity of the material that makes up the shaft. This varies depending on the material, and the values of G for many kinds of materials can again be found in charts in design handbooks and from manufacturers. For this situation, assume G = 90 Gpa. Note that G is also the shear stress, τ, divided by the shear strain, γ, or τ/γ.
So for this example,
Therefore, θ, the angle of twist is 0.0543 radians or 3.111°. Performing these calculations using the system’s needed torque is necessary to determine the angle of twist for those conditions. Make sure to look up the correct equations for J depending on the type of shaft and for G depending on the type of material.
5. How does frequent disengagement affect mechanical couplings?
Frequent engagements and disengagements of mechanical couplings can be detrimental. The rubbing together of their surfaces causes friction, which can subsequently cause wear and eventual failure of couplings. This abrasive operating condition means such setups have inherently limited lifespans.
There are a few workarounds to this abrasive operation. One is limiting the number of engagements and disengagements a coupling experiences. This is best accomplished by optimizing the system’s operation and making sure no unnecessary starts and stops are present during operation.
Such operating conditions also require proper lubrication. Thoroughly read all documentation and make sure that enough lubrication is present and it is the right type required by the coupling in question.
Another option is to use contactless magnetic couplings. These use a magnetic field to connect to elements without mechanical contact. This removes the abrasion and wear from contact and allows for smooth engagements and disengagements. This solution works best when magnetic fields are of no concern in a system. Furthermore, magnetic couplings’ torque properties are not identical to traditional couplings, so keep this in mind for engineering calculations.