by Bill Hewitson, Ruland Manufacturing Co.
In the design of motion control equipment, the coupling is often left as one of the last components to be specified and is often overlooked or taken for granted rather than treated as a critical part of the performance of the system. Selection of the proper coupling ensures the equipment will meet the requirements for performance and have a long, trouble free life. Conversely, poor coupling selection can leave designers and end users of equipment frustrated with imprecise positioning and frequent maintenance requirements.
Many types of couplings are available for motion control applications, each with their own pros and cons. Beam couplings are a good starting point for designers because they do many things well: they have good misalignment capabilities, torsional stiffness for accuracy, and strength for carrying torque loads that allow them to be used in many applications. There are several different design considerations that allow users to optimize characteristics to meet their needs, such as material differences and beam pattern designs.
Criteria to consider for selecting a coupling
There are some basic criteria that designers should gather and understand about their application before specification of a coupling can take place.
Physical space requirements: Basic information such as shaft sizes, spacing between shafts, shaft geometry (round, D-bore, keyed, hex, polygon, and so on) and overall envelope size are the first considerations for specifying a coupling. Special consideration should be paid to designs that have large differences in shaft sizing between driving and driven components and large spacing between shafts and small windows to fit a coupling. Applications that involve inch and metric shafts and non-standard shaft sizing/tolerancing can also present issues in selection of a coupling.
Misalignment: Shaft misalignment is one of the most critical pieces of information needed to correctly specify a coupling. Often times, this information is not given proper consideration, resulting in the failure to calculate tolerance stacks and manufacturing inconsistencies adequately. Consequently, couplings can be misapplied in applications where shaft misalignment is greater than the coupling can accommodate, resulting in poor performance and frequent maintenance. Angular and parallel misalignment, as well as axial motion (whether from push/pull motions, thermal changes or simply play from bearings), must be considered together.
Environmental considerations: Most motion control applications exist in controlled climates without extremes in temperature, moisture, chemicals or other environmental factors that can affect the coupling performance. However, some applications encounter conditions that are less than ideal. Extreme temperatures, both hot and cold, can decrease performance of beam couplings—material strength and elasticity/brittleness can be negatively affected. Other exposure possibilities include the presence of chemicals that could compromise the integrity of the material, operation within a vacuum with out-gassing concerns, or electrical current passing through the coupling (whether intentional or incidental).
Operating conditions: Many factors exist that designers need to be aware of when selecting a coupling. Conditions such as speed (rpm), acceleration/deceleration rates, rotational cycling (continuous, start-stop, reversing), and duty cycle (intermittent, continuous, 24/7, 8 hrs/day) are critical to coupling selection. Torque must also be understood when specifying the coupling—not simply rated torque, but real torque seen by the system. Inertia loads, especially in systems with rapid acceleration/deceleration curves and systems that encounter hard mechanical stops, can have unexpectedly high real torques seen by the coupling.
Performance criteria: Finally, performance expectations from the system must be considered. Level of positioning accuracy and repeatability, amount of settling time that is tolerable when the system reaches position, and overall responsiveness of the system should all be taken into account. Additionally, consider factors such as dampening in the system against shock loading, need for electrical isolation, and overall expected life of the system, and associated maintenance schedules.
Once all of these factors have been investigated and understood, selection of the proper coupling can begin. Understanding the different types of beam couplings that are available and their relative strengths and weaknesses will help in the coupling selection process.
Available beam coupling designs
Two basic designs of beam couplings exist: single beam and multiple beam, both of which are machined from a single piece of material. Single beam designs consist of one long continuous beam that generally is several complete rotations in length. The long beam yields good angular and axial flexibility; however, it is not nearly as effective with parallel misalignment. To accommodate parallel offset, the single beam is forced to simultaneously bend in two different directions resulting in a large stress within the beam that can rapidly cause fatigue and subsequent failure of the coupling.
The long single beam also suffers in torsional stiffness capabilities. Its benefit of flexibility under misalignment-derived forces is a weakness in the degree of positional accuracy it can deliver. Under torque loads, especially during high rates of acceleration or deceleration, the coupling “winds-up” like a spring, causing an angular displacement from one end of the coupling to the other. This windup results in a positioning lag between the driving and driven side of the assembly while in motion, thereby negatively affecting the accuracy of the driven end of the system.
If the system requires precise positioning throughout the entire path of motion, a single beam coupling may not be the best fit. Additionally, torsional stiffness is a factor when motion comes to a stop. The angular lag of the coupling can force the servomotor to “seek” its position, causing an oscillation back and forth until the coupling can reach its original free length and the system comes to rest. Long settling times greatly affect the speed and accuracy of the system.
Multiple beam couplings have a slightly different design to improve performance when compared to single beam couplings. Several basic designs currently exist with different performance characteristics.
The first design consists of a single set of overlapping beams (typically three beams) as opposed to a continuous single beam. The multiple beam design allows the coupling to have beams that are shorter in length, which is beneficial to positioning accuracy. The shorter the length of the beam, the stiffer and stronger it is under torsional loads. Shorter beams, however, are also stiffer under misalignment loading, which reduces the overall ability of the coupling to compensate for manufacturing tolerances in application.
The concept of nesting multiple short beams together into a single set of beams improves the ability of the coupling to accommodate all misalignment types. It still is not as flexible as a continuous single beam coupling, but has enough flexibility to be used effectively in almost all applications. The set of beams also creates greater strength due to having the beams work in parallel with each other to achieve higher torque capabilities. This configuration still suffers from issues with parallel misalignment and is even more susceptible to rapid fatigue do to the shorter, stiffer beams.
A further enhancement to the multiple beam design is incorporating multiple sets of multiple beams into a single coupling. Multiple sets of cuts add an extra dimension to the performance of beam couplings—the ability to easily handle parallel misalignment conditions. A beam coupling with a single set of cuts compared to one with two sets of cuts can best be compared to a single cardan universal joint versus a double cardan universal joint. A single joint easily handles angular misalignment, but not parallel, while a double joint handles both types of misalignment easily. However, unlike universal joints, whether a single or multiple beam coupling, all designs offer the benefit of constant velocity without the phasing associated with universal joints.
There are designs that use two sets of long, continuous single beams, but most designs feature multiple beams in each set. Two sets of multiple beams provide the best overall performance in beam couplings. Ruland manufactures two different varieties of this design to meet the needs of many different applications.
The first design uses two sets of two beams for a total of four beam couplings (MWC/MWS, PCR/PSR) and is available in inch, metric and inch-metric shaft sizes as standard items from stock ranging from 3 to 12 mm (3/32 to 1/2 in.). The four beam couplings have a compact design that allows for easy interchange with many single beam couplings and fits easily into confined spaces. These are designed as lighter duty couplings with greater flexibility that are best used in low torque applications, such as encoders, tachometers and other instrumentation, where precise positioning is required.
The length of the beams is kept relatively long (although much shorter than single beam couplings—approximately 1.5 rotations vs. 3 or more) to maintain flexibility in the coupling and dampen vibration and shock loads in the system. Flexibility allows for greater tolerance in installation and manufacture without causing premature failure of the coupling and protects expensive components from excessive bearing loads created by reactionary forces due to misalignment of shafts. The shorter beam design also allows these couplings to achieve higher torque capabilities and torsional stiffness in a size that fits in the same overall envelope.
The second design involves two sets of three beams for a total of six beam couplings (FCR/FSR), which are also available from stock in inch, metric and inch-metric shaft sizes from 3 to 20 mm (3/16 to 3/4 in.). The use of three beams in each set further allows for shortening of beams. This series uses beams that are only one full rotation in length, resulting in higher torsional stiffness and torque capabilities. It also raises the natural frequency of the coupling, giving the designer opportunity to tune the system to higher performance levels. The stronger beams are combined with larger body sizes, resulting in a coupling that is best suited for light duty power transmission applications, such as connecting a servomotor to a ball screw. Bearing loads are higher than the two beam design, so these are best used in applications with components that tend to have more durable bearing support, such as servo motors and linear stages. This series also features Nypatch anti-vibration treatment on clamp screws to prevent screws from loosening during use.
Beam coupling material selection
Material is a factor that should be considered early in the specification of a beam coupling. The specified material determines a vast majority of the performance characteristics of the coupling because it is machined from a single solid piece. Typically, beam couplings are manufactured from either aluminum or stainless steel, but are also available in plastics such as Delrin, titanium and other engineering grade materials.
Aluminum is the most common choice for material because it has a good mix of attributes for many beam coupling applications. Aluminum is lightweight and has an excellent strength to weight ratio. Low weight allows the couplings to be designed with low inertia to allow for a high level of responsiveness in the system without having the coupling diminish this performance characteristic. Aluminum’s strength and fatigue resistance allows long, service free life, even in demanding applications.
Stainless steel is the next most common choice for beam couplings. Most frequently, stainless steel is chosen to give increased strength to a coupling. Stainless steel couplings are typically about twice as strong as an aluminum coupling of the same design and have high torsional stiffness. The primary drawback to stainless steel is that it has a mass almost three times that of aluminum, which results in relatively high inertia that can diminish the responsiveness of the system. For this reason, designers should be cautious of installing stainless steel couplings in motion control systems. The best applications for stainless steel are ones that encounter environmental issues such as weather, temperature extremes, chemicals or exposure to vacuum.
Other materials, such as titanium and Delrin, are far less commonly used. Typically, these are specialty applications and require special designs. The costs and difficulty of machining keep titanium from being a viable option except where absolutely necessary to the application—typically aerospace.
Delrin has some advantages in that it has less mass and inertia than even aluminum and has the additional benefit of electrical isolation, which can be important for applications with components that can be damaged or suffer interference from stray currents passing through the coupling. However, Delrin has very low strength and can only be suitably used in applications with extremely low torque requirements. A better option for applications requiring isolation is to use aluminum couplings with insulating inserts. This has the added benefit of isolation without sacrificing coupling strength, making the combination well suited for many applications.
Filed Under: Design World articles, Couplings, Mechanical