Backlash-free couplings for high-tech applications

For high technology applications such as test and measurement, inspection, semi-conductor, solar panel production equipment as well as medical devices, Ruland manufactures a variety of components including oldham couplings and beam couplings. While both coupling types are backlash-free and feature misalignment capabilities, small performance differences make the choice of coupling critical depending on application parameters.

Ruland oldham couplings are comprised of two aluminum hubs with drive tenons that mate with a floating center member. This design allows for easy sliding to accommodate misalignment. Oldham couplings have very light bearing loads since the only resistance caused by misalignment is frictional. They also have the advantages of electrical isolation. The couplings accommodate angular and axial misalignment, but are especially useful in applications where parallel misalignment is present. The replaceable center discs are available in two materials, offering different levels of torsional rigidity.

Beam couplings have overlapping spiral cuts in a single piece of aluminum.  The spiral cuts accommodate angular misalignment, parallel misalignment, and axial motion. The one-piece design and cut pattern ensure backlash-free operation and allow for a small amount of shock absorption. Beam couplings are especially suitable when angular or complex misalignment is present. Stainless steel versions are also available for applications requiring higher torque transmission capabilities.

Ruland beam and oldham couplings are offered in a variety of body sizes with bore sizes from 3/32 in. (2 mm) to 1-1/8 in. (28 mm).

The complete line of products includes shaft collars and rigid couplings, and a full line of motion control couplings: beam couplings, bellows couplings, oldham couplings, miniature disc couplings and zero backlash jaw couplings.

Ruland Manufacturing Co., Inc.
www.ruland.com/ps_couplings_flexible.asp
www.ruland.com/ps_couplings_oldham.asp

Shoulder Couplings Ideal For Abrasive Surfaces

Victaulic now offers a new line of products for joining standard shouldered steel pipe. The product line features the Style SC77, a flexible coupling featuring Victaulic patented installation-ready technology, and a variety of prefabricated shouldered fittings designed to make pipe joining even more efficient. The product line is ideal for abrasive services, particularly in the mining industry, where maintaining full pipe-wall thickness is important.

www.victaulic.com

Six factors to remember about couplings in a motion system

Physical values such as torque, torsional rigidity, spring stiffness, moment of inertia, imbalance, and zero-backlash play a major role in coupling design. Here are a few facts to keep in mind when you design your motion system.

Torque (Nm): is the product of an acting force and the effective length of the acting force’s lever arm.

T = Fxr

T = Torque (Nm)

F = Force (N)

r = Lever arm (m)

With a force of 100 N and a 1 m long lever arm, you can generate a torque of 100 Nm. Or, you can generate a torque of 100 Nm with a force of 1000 N and a 0.1 m long lever arm. For couplings, a specific amount of torque can be achieved with a large outer diameter of the coupling and a correspondingly low acting force or with a small outer diameter and a correspondingly high acting force.

Torsional rigidity (Nm/rad): refers to the rigidity of a coupling when it is subjected to a torsional load. If the torque exceeds the maximum torsional value of the coupling, the coupling will no longer be strong enough to transmit the acting rotational force. Ex: If a coupling with a torsional rigidity of 10 000 Nm/rad is subjected to 10 Nm, the connection element will twist by 1/1000 rad. That is equal to an angle of twist of about 0.057 degrees (1 rad = 57°17’44.8”). For a torsionally rigid or vibration damping coupling, this angle of twist may still be within the admissible range.  In practice, torsionally rigid couplings normally have a maximum angle of twist of less than 0.05 degrees and vibration damping couplings have a maximum angle of twist of less than 5 degrees.

Spring Stiffness (N/mm): is the counterforce exerted by the coupling in case of differentiated position of the axes in an axial, radial, and lateral direction. Ex: If the axial spring stiffness of a coupling is 30 N/mm, the coupling will exert a force of 30 N in the case of an axial displacement of 1 mm. These forces are important in a design with couplings, particularly when selecting bearings or other drive system components.

Moment of inertia: is the moment resistance when the rotational speed is changed. Normally, the lower the total weight and the smaller the outer diameter of the coupling body, the lower the moment of inertia. The reverse is also true, the higher the weight and larger the outer diameter, the higher the moment of inertia. This feature is important in highly dynamic applications because the drive has to generate sufficient torque to overcome a body’s moment of inertia to accelerate and decelerate.

Imbalance: in a drive system, imbalance should be as low as possible for smooth operation. Caused by asymmetries in the drive system where mass is distributed unevenly, it affects centrifugal forces on the entire drive system. It can be rectified by “balancing bores,” which are normally drilled directly into the location of the disproportionally high concentration of mass.

Zero backlash: is a lack of empty space or “play” when the rotational speed, direction of rotation, or torque changes. It does not mean that there is no angle of twist. Backlash is an important factor in predicting bearing life.

Information courtesy of R+W America

Rw-america.com

EZV Series Adjustable Line Shafts from R+W America

A convenient location for manual phase adjustment along a mechanical drive system is now available in the EZV series adjustable line shafts. Making use of a high strength intermediate collar between two telescoping sections of precision tubing, the EZV naturally places the location for phase adjustment in an easily accessible, open space. Due to the relatively large outside diameter of the drive tubing, the EZV also provides for a more secure clamping connection than would exist when clamping over standard diameter motor and gearbox shafts.

Length adjustability also results from this design, making the EZV reusable in different machine layouts, and easier to install, especially with certain alternate hub designs, like EK7 expanding mandrels and EK6 high strength conical clamp ends. For any size the EZV can also be made with ZAE torsionally rigid bellows couplings, or with integral ES2 mechanical torque limiters.

R+W America
www.rw-america.com

Rebar Coupling Saves Time & Money

The LENTON^® LOCK S-Series, part of the LENTON LOCK Mechanical Rebar Splicing System from ERICO^®, provides many of the same great benefits as the B-Series, except in a smaller package. This unique bolted splice is smaller than the competition and has fewer bolts, which helps reduce installation time and save money on the jobsite. All LENTON LOCK couplers feature patented gripping technology that provides overall structural integrity in tension, compression and stress-reversal applications.

With a performance that exceeds 125% of specified yield on ASTM^® A615 Grade 60 rebar, this innovative mechanical rebar splice is designed for use in column splicing, bridge applications, piling, splicing to protruding dowels cast in concrete, closure pours, beams, chimney construction and other demanding splicing applications.

The couplers allow for easy field installation since no bar-end preparation, sawing or swaging is necessary, and they can be installed with just a standard wrench or an impact wrench, depending on coupler size. The bolt heads will shear off when proper installation tightness has been reached, which allows for complete visual inspection.

The S-Series is ideal for use in repair, bent bar, retrofit, precast closure pour and new construction applications. The couplers meet or exceed major international building codes and Department of Transportation requirements, including ACI^® 318 Type 1, UBC^® Type 1 and IBC^® Type 1.

There is no need to stock multiple parts to accommodate one-step transitions. LENTON LOCK works as a one-step transition on ASTM (in-lb) rebar. For Canadian rebar sizes, contact ERICO for transition compatibility.

www.erico.com

Tips to simplify coupling selection

For a coupling in a servo application to work properly, you need to satisfy a number of application factors including: torque, shaft misalignment, stiffness, speed, and space requirements.

Here’s a look at the available types of servo couplings and what you need to consider for each of them during the selection process.

Beam couplings
Beam type couplings are manufactured from a single piece of material, usually aluminum, and use a system of spiral cuts to accommodate misalignment and transmit torque.  For many applications, beam couplings are a good economical and maintenance free choice.

The single piece design transmits torque with zero backlash. Two basic variations exist: a single beam style and a multiple beam style.

The single beam style has one long continuous cut that usually consists of multiple complete rotations. It is very flexible and accommodates light bearing loads.

For many applications, flexible beam couplings are a good economical and maintenance free choice.

It is able to manage all types of misalignment, but works best with angular misalignment or axial motion. It is not well suited to parallel misalignment because the single beam must bend in two different directions simultaneously, creating larger stresses in the coupling that could cause premature failure.

Under misalignment conditions, the long single beam allows the coupling to bend easily. But the relatively large amount of windup under torsional loads adversely affects the coupling’s accuracy.

Single beam couplings are an economical option best used in lower torque application and in connections to encoders and other light instruments.

Multiple beam couplings, which usually consist of two or three overlapping beams, attack the problem of low torsional rigidity. The use of multiple beams lets the beams be shorter without sacrificing much of the misalignment capabilities.

The shorter beams make the coupling torsionally stiff. Overlapping them so the beams work in parallel increases the allowable maximum torque making them suitable for use in light duty applications with connections, such as from a servo to a leadscrew. A drawback is that bearing loads are increased by a sizeable amount over the single beam variety but, in most cases, remain low enough to protect bearings effectively.

Some manufacturers take the multiple beam concept to another level. Instead of using a single set of multiple cuts, they use two sets. The use of multiple sets of cuts gives the coupling additional flexibility to accept more misalignment, including parallel misalignment. With parallel misalignment, one set of beams bends in one direction and the second set bends in the other direction.

Most commonly, these couplings are made of aluminum, but they also come in stainless steel. Stainless protects against corrosion, and increases coupling torque capacity and stiffness to sometimes double that of aluminum versions. The increase in torque and stiffness, though, is offset by a dramatic increase in mass and inertia. Keep in mind that in applications using smaller motors, a large percentage of the motor’s torque is used to overcome the inertia of the coupling.

Oldham couplings
The Oldham coupling is a three piece coupling comprised of two hubs and a center member. The center disk, which is usually made of a plastic or, less commonly, a metallic material, transmits the torque. On the center disk, mating slots are located on opposite sides and oriented 90 degrees apart. Drive tenons are located on the hubs. The slots of the disk fit on the hub tenons with a slight press fit that allows the coupling to operate with zero backlash. Over time, the sliding of the disk over the tenons will create wear to the point where the coupling will experience backlash. The disks are inexpensive items easilyreplaced, so a new insert will restore the coupling’s original capability.

The choice of materials for Oldham couplings depends on requirements for backlash, stiffness, vibration, and noise.

In operation, the center element slides on the hub tenon to accommodate misalignment.

The only resistance to misalignment is the frictional force between the hub and disk, Oldham couplings have bearing loads that do not increase as misalignment increases. Unlike other types of couplings, there are no bending members that cause bearing loads to increase as the shafts get out of alignment.

These couplings only allow a small amount of angular misalignment (less than one-half a degree) and axial motion (less than 0.005 in.), and are limited to speeds of 4000 rpm. Larger amounts of angular misalignment cause the coupling to lose its constant velocity characteristic, and axial motion is limited by the three-piece design of the coupling, which does not allow for use in push-pull types of applications. Because the center disk is a floating member, both shafts must be supported to keep the coupling from falling apart.

Bellows couplings easily bend under loads that result from angular, parallel, and axial motion.

Oldham couplings can handle relatively large amounts of parallel misalignment, from 0.025 in. to 0.100 in. or more depending on coupling size. Coupling manufacturers generally provide smaller misalignment ratings to obtain longer life ratings. These ratings can be surpassed at the expense of coupling life.

These couplings are available in a range of disk materials. The choice depends on requirements for zero backlash, high torsional stiffness and torque, or vibration absorption and low noise. Nonmetallic inserts are electrically isolating and can act as a mechanical fuse. When the plastic insert fails, it breaks cleanly and does not allow transmission of power, preventing other damage from occurring to machinery components.

Zero backlash jaw couplings
Jaw couplings are either conventional straight jaw or curved jaw zero backlash versions. Conventional straight jaw couplings are not typically well suited to servo applications that require the accurate transmission of torque. Zero backlash jaw couplings, on the other hand, are well suited to servo applications. The curved jaws help to reduce deformation of the spider and limit the effects of centrifugal forces during high-speed operation.

Jaw couplings handle high-speed applications well, but are less able to handle large amounts of misalignment.

Zero backlash jaw couplings consist of two metallic hubs and an elastomer insert commonly referred to as a “spider.” The spider is a multiple lobed insert that fits between the drive jaws on the coupling hubs with a jaw from each hub fitted alternately between the lobes of the spider. As in the oldham coupling, there is a press fit between the jaws and the spider for the coupling to deliver zero backlash.

In contrast to the oldham coupling, where the torque disk is in shear under torsional loads, the jaw coupling’s spider operates in compression. Be careful not to exceed the manufacturer’s rating for maximum torque, which can be significantly below the physical limitations of the spider. The spider can be compressed so that there is no longer a preload and backlash will occur.

Jaw couplings are well balanced and able to handle high-speed applications, 40,000 rpm or more. They do not handle very large amounts of misalignment, especially axial motion. Large amounts of parallel and angular misalignment cause loads on bearing to be higher than those of most other types of servo couplings.

If a spider fails, the coupling will not disengage. The jaws from the two hubs will mate similar to teeth on two gears and continue to transmit torque with metal-to-metal contact. Depending on the application, such action may be desirable or it could cause problems in the overall coupling system.

An advantage of the jaw coupling is the ability to mix and match spiders based on the application. Manufacturers of zero backlash jaw couplings offer multiple materials with different hardnesses and temperature capabilities that let you choose exactly the insert that meets the application’s performance criteria.

Disk couplings
At minimum, disk couplings have two hubs and a thin metallic or composite disk that transmits the torque. The disk is fastened to the hubs usually with a tight fitting pin that eliminates any play or backlash between the parts.

Torsionally stiff, disc couplings can accept up to 5 degrees of misalignment with some of the lowest bearing loads available.

Some manufacturers offer disk couplings with two disks separated by a rigid center member attached to a hub at each end. The rigid center member is usually metallic, but plastic versions are available and can be used to electrically isolate the coupling. This configuration will reduce torque capacity and torsional stiffness.

The difference between the two variations is similar to the difference between the single beam style coupling and the multiple beam coupling with two sets of cuts. The single disk coupling is not adept at accommodating parallel misalignment due to the complex bending of the disk. The two-disk style allows each disk to bend in opposite directions to harness the parallel offset. The properties of this type of coupling are similar to those of bellows couplings. They transmit torque in a similar manner. The disks are very thin, allowing them to bend easily under misalignment loading, which allows the coupling to accept misalignment up to 5 degrees with some of the lowest bearing loads available in a servo coupling.

Torsionally, the disks are very stiff. The disk coupling has stiffness ratings slightly lower than that of bellows couplings. A downside to these couplings is that they are delicate and prone to damage if misused or installed improperly. For prop