Zero Backlash Coupling Ideal for Belt Driven Linear Actuators

Due to greater demands in modern machine design and construction, R+W has developed its EK7 series of SERVOMAX Zero Backlash Elastomer Couplings.

This coupling was specifically designed to mount to hollow bores with an expanding tapered clamping element, making it ideal for belt driven linear actuators. The EK7 provides design engineers with a number of advantages, including:

· Most belt driven linear actuators require a shaft adapter in order to couple the pulley to a motor or gear head. The EK7 eliminates the need for this additional hardware.

· In designs where motor shafts are typically mounted directly into the pulley, customer specified products often provide shafting that is too large. The EK7 can couple to the large shaft with a smaller expanding element to link the two together.

· Assemblies involving a motor, gear head and coupling used to drive the actuator can become quite large and cumbersome. The EK7 plays a small part in reducing that system size by reducing the coupling adapter flange by the length of one hub.

In addition to providing these and other novel mechanical linkage solutions, the EK7 also offers the benefits of all SERVOMAX couplings, particularly in its zero backlash torque transmission. The moment of inertia is also very low due to its low mass and low weight; ideal for high-speed servo applications where rapid acceleration/deceleration cycles exist.

The couplings are manufactured with precision machined jaws and an elastomer insert press fit between them for vibration damping and zero backlash transmission of torque. The coupling hub is custom bored and can accommodate shaft diameters from 4 to 60 mm (3/16 to 2.25 in.) and the expanding hub can accommodate bores from 12 to 70 mm (1/2 to 2.75 in.). Sizes are available for torque capacities up to 450 Nm (4,000 in. lbs.).

GERWAH® Product Line

The GERWAH® line of products consists of magnetic couplings, metal bellows couplings, servo-insert couplings, line shafts, RING-flex® couplings and safety couplings. These couplings are available in a range of sizes and torque capacities to 3,800 lb-ft. The low mass of the lightweight construction helps increase machine performance and reduce energy costs.

Ringfeder Power Transmission USA Corporation markets a range of power transmission components and keyless shaft/hub technology.  Other power transmission products include shock absorbing devices, flexible elastomeric couplings, flexible disc couplings and torque limiters along with other specialty and custom made products.

RINGFEDER

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Elastomer couplings with higher torque handling capacity

The growing popularity of curved jaw (elastomer) style couplings for precision applications has driven the need for couplings that handle more than the traditional torque capacity of 2,150 Nm up to a maximum torque of 25,000 Nm.

Available with split clamping collars or keyway and set screw connections, the three new body sizes allow for backlash free, vibration damping power transmission, paired with strong torque density. Dual flexture and jack shaft versions are also available for spanning longer distances and compensating for larger misalignments. Unlike the pre-existing range of R+W elastomer couplings, which use a single spider element between the new hubs, the new larger sizes will use individual vibration damping compensation elements to fit between each mating set of coupling teeth. These couplings are available in English and metric bore diameters up to 170 mm.
R+W America
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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

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For safety, electronics may not be the best choice

The trend of replacing mechanical systems with electrical systems continues. Even developers of hydraulic and pneumatic systems are following it. But, as is becoming evident through the latest unintended acceleration issues, electronic components can have a few drawbacks that should not be overlooked in a design.

When in comes to designing a system for safety, specifically when considering whether to choose a mechanical component such as a coupling, or to go electronic, remember this: Electronic safety components have two major disadvantages compared to mechanical safety components.

1. Reaction time. Assume a machine crashes and causes an overload. According to engineers at R+W America, a signal from the monitoring circuit does not reach the motor controller until 5 to 7 ms following a sharp increase in torque. During this period of latency, the controller attempts to further increase torque to reach the setpoint value. Most likely, another 10 ms will pass before the motor is shut off. Depending on the drive train’s moments of inertia, more time can pass before the electronics brings the whole system to a stop.
2. Multiple potential failure sources. Electronic monitoring systems need multiple sensors for data. Between the monitoring system and all of its sensors and other components, you have a system with multiple possible points of failure.

A mechanical safety coupling, on the other hand, completely disconnects the drive from the load within 3 to 5 ms; 1/3 of the time needed by an electronic cut-off. Noted engineers at R+W America, “electronic machine monitoring is not suitable for high speeds due to the large centrifugal mass of the rotating parts.”

Also with a mechanical safety coupling, you have one component per axis, reducing the number of possible points of failure.

Safety couplings must demonstrate two clear behaviors:

1. Upon overload, separation of drive train and load should occur within a few milliseconds.
2. After the coupling has disengaged, residual friction should not be excessive so as not to damage coupled components that continue to be driven due to mass moments of inertia.

According to R+W, safety couplings can be subdivided into five classes:

1. Rigid safety couplings used in indirect drive applications.

2. Torsionally rigid safety couplings for use between two shafts or flanges. These couplings resist twisting and can be subdivided into two groups.

A. Single-piece torsionally rigid safety couplings.

B. Press-fit couplings.

3. Vibration-damping safety couplings are fitted with an elastomer insert that damps incurred drive vibration.

4. Economy safety couplings suit applications requiring simple overload protection and functions as a variation of the ball-detent principle.

5. Torque-limiting line shafts, which span long distances between shafts.

(Some material, courtesy of R+W America.)

Thermoplastic Insert Couplings from R+W America

R+W has developed the ECOLIGHT- TX1 series of molded thermoplastic insert couplings. The couplings feature a dramatically reduced mass and moment of inertia, accompanied by a high level of chemical compatibility and tolerance for heat, making the TX couplings ideal for both pump drive and servo applications. These plastics also provide major cost reduction compared to metals, as the result of decreased production times and relatively low material cost.

The material of the hubs is a glass fiber reinforced plastic developed for rigidity and corrosion resistance. Accelerated life testing involved 40 million load reversals at the coupling rated torque. Zero change to the integrity of the coupling keyway was discovered, as the steel key itself was the more brittle and compliant of the two materials. The thermal stability of the material is also critical, as an H7 shaft fit tolerance for the hub must be maintained regardless of the environments in which the couplings will be used.

R+W TX couplings have an elastomer element that is press fit between the two precision molded coupling halves giving it zero backlash and guaranteed concentricity. The elastomer insert is available in several Shore hardness values allowing the designer to choose how great of a damping effect is required for the application. The TX1 series coupling is designed for a pure keyway connection and is molded or bored to customer shaft specifications. Shaft mounting is achieved via a keyway and a set screw.

Sizes are available for torque capacities ranging from 2Nm (17.7 lbs/in) to 660Nm (5841 lbs/in) and bore diameters from 8 to 45mm and .375 to 1.75 inch with standard or metric keyways. Special shaft interface, such as spline hubs are also available upon request.

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Expanded Line of Servo Insert Couplings

The SERVOMAX line of precision elastomer insert couplings from R+W has been expanded to include both smaller and larger sizes as well as enhanced mounting options. The R+W SERVOMAX utilizes secondary heat treatment of the injection molded polyurethane insert to smooth out inconsistencies in the driving surfaces, allowing for a smooth fit and higher repeatability.

The couplings compensate for all types of shaft misalignment while damping vibration and maintaining a backlash free performance. Three different Shore hardnesses allow the designer to select the correct amount of flexibility, vibration damping or stiffness depending on their performance requirements. Their high torque density and reasonable cost makes them ideal for aggressive applications within a small space.

The newly redesigned SERVOMAX EK range now includes 9 sizes and accepts English or metric shafts ranging from 3 – 80 mm (0.118 – 3.150 in.). Torques range from 0.5 – 2150 Nm (4.43 – 19,027 in-lbs). Available in aluminum, steel, stainless steel or engineered polymer, new mounting styles include a wear resistant mechanical torque limiter, fully split clamping collars, adjustable length drive shafts and more. For more information contact R+W America at (630) 521-9911 or info@rw-america.com

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