Ruland Servo Coupling Selection Guide

Servo couplings are specialized shaft couplings designed for use in servo motor applications. Pacific International Bearing Sales Inc. offers Ruland servo couplings in the following designs. Ruland Beam Couplings, Bellows Couplings, Controlflex Couplings, Disc Couplings, Jaw Couplings, Oldham Couplings, and Ruland Slit Couplings.

Servo couplings are critical components in motion control and industrial automation systems, connecting servo motors to driven components with high precision. The right coupling ensures zero backlash torque transmission while accommodating inevitable shaft misalignments, protecting motor bearings, and maintaining accuracy. Ruland Manufacturing offers a full range of high-performance servo couplings, including beam and bellows couplings, as well as disc, jaw, and Oldham styles, each with unique advantages. 

This guide offers technical selection advice for Ruland servo couplings, including part numbers and specifications, to help engineers select the optimal solution for their production applications. Pacific International Bearing Inc. (PIB) is an authorized distributor of Ruland couplings, offering expert support and convenient online ordering.

Before diving into coupling types, consider the key factors in selecting a servo coupling for your application:

• Torque and torsional stiffness. Ensure the coupling can handle the peak torque without wind-up (important for positioning accuracy).
• Misalignment accommodation. Determine the types and magnitude of shaft misalignment (angular, parallel, axial) that the coupling needs to handle.
• Speed and vibration. High-speed motion may require a balanced coupling design to minimize vibration; some couplings also offer vibration-damping.
• Inertia and size constraints. Choose a coupling with suitable mass and size – lower inertia couplings improve servo response.
• Environmental and maintenance needs. Consider if electrical isolation is needed, or if the coupling has wearable elements (like inserts) that require periodic replacement.
  

Below, we review each Ruland servo coupling type, including how they work, the technical benefits, and selection guidance, along with part number examples.

Beam Couplings (Helical Couplings)

Beam couplings (also known as helical couplings) are one-piece flexible couplings machined from aluminum or stainless steel with spiral cuts. The helical cut pattern allows the coupling to flex and compensate for misalignment while transmitting torque. Ruland’s beam couplings operate with zero backlash and no maintenance, making them a versatile general-purpose choice for servo systems.

There are two major beam coupling styles:

• Single-beam couplings: These have one continuous spiral cut that goes through the length of the coupling multiple turns. A single long cut makes the coupling very flexible with minimal restoring force. It easily handles angular misalignment and axial motion, and exerts light bearing loads. However, a single-beam design is less effective for parallel misalignment (since the beam must bend in two directions simultaneously) and it is less torsionally rigid. Single-beam couplings are best suited for low-torque applications such as small encoders, tachometers, or other light instrumentation.
  
• Multiple-beam couplings: These use multiple overlapping helical cuts (for example, two sets of cuts oriented 90° apart) to increase torsional stiffness and torque capacity. By shortening and overlapping the beam sections, the coupling becomes more rigid and can transmit higher torque with less wind-up. It also improves parallel misalignment capability, as one set of beams flexes in one plane and the other set in the perpendicular plane. The trade-off is slightly higher bearing loads than a single-beam design (due to the stiffer beams), but in practice, the bearing loads remain low enough for most servo motor bearings. Multiple-beam couplings are often the starting point for servo motor to leadscrew or light-duty drive connections where moderate torque and misalignment must be balanced.
  

Ruland manufactures three series of beam couplings to cover a range of performance needs:

• F-Series (six-beam): High flexibility couplings with longer multiple cuts (two sets of three beams) for maximum misalignment accommodation and lowest bearing loads.
• P-Series (four-beam): Higher torsional rigidity and torque capacity thanks to a shorter four-cut design – ideal for precision positioning with moderate misalignment.
• MW-Series (metric six-beam): Fully metric-dimensioned couplings similar to F-series, for designers using all metric hardware.
  

Beam couplings are offered in clamp or set-screw hub designs. They come in bore sizes from as small as 3 mm (1/8″) up to about 20 mm (3/4″), covering shafts from tiny encoders to mid-sized servo motors. For example, part number PCMR16-5-5-A denotes a Ruland P-Series clamp-style beam coupling (metric) with a 16 mm body diameter and two 5 mm bores in aluminum. Stainless steel versions (identified by an “SS” suffix) are available for higher corrosion resistance and about twice the torque capacity (with some increase in inertia).

Overall, beam couplings provide a good balance of flexibility and precision at a relatively low cost. They have zero backlash and a balanced design for smooth rotation at high RPM. If your servo application has moderate torque and requires accommodation of all misalignment types (especially angular and axial) without extremely high rigidity, a beam coupling is often an excellent choice.

Bellows Couplings

Bellows couplings are the most torsionally rigid servo couplings in Ruland’s lineup. They consist of two metal hubs (typically anodized aluminum) joined by a thin-walled metal bellows made of stainless steel. The bellows’ accordion-like structure below flexes to accommodate misalignment while remaining rigid under torsional (twisting) loads. This gives the below couplings an exceptional combination of zero backlash operation and extremely high torsional stiffness – ideal for precision servo axes that demand accurate positioning.

Ruland bellows couplings can handle the three types of misalignment (angular, parallel, and axial), but only in small amounts. The thin bellows walls bend easily for slight misalignments (keeping bearing loads low and nearly constant through rotation), but excessive misalignment will stress the bellows. In practice, bellows couplings are best for well-aligned systems where misalignments are minor (a few thousandths of an inch or less). They excel at maintaining constant velocity and precise motion transfer. Because the coupling is all-metal, there is no damping of vibration or shock; bellows couplings will transmit torsional shocks directly to the system.

Ruland offers bellows couplings in various sizes and configurations, including inch, metric, and inch-to-metric combinations. Standard models are balanced for high-speed operation up to 10,000 RPM. Engineers can select short or long bellows versions: a longer bellows can accommodate a bit more misalignment, whereas a shorter bellows yields the highest rigidity. Hubs are clamp-style (with or without keyways) to ensure a secure, concentric grip on the shafts. Bore diameters range roughly from 3 mm up to 32 mm (1/8″ to 1-1/4″). For example, MBC41-16-16-A is a Ruland metric bellows coupling with a 41.3 mm OD, 16 mm bore on each side, made of aluminum hubs and stainless bellows.

In terms of performance, bellows couplings provide zero backlash, zero maintenance operation. They have the highest torsional rigidity of any flexible coupling type, which means they minimize positioning error in high precision motion control. This comes with a higher price point and the requirement of near-perfect alignment. Use bellows couplings for applications such as servo-driven indexing tables, precision gantry systems, or metrology equipment, typically when accuracy is prioritized over misalignment accommodation or shock absorption.

Disc Couplings (Single and Double Disc)

Disc couplings use thin, flexible discs of spring steel to transmit torque. Ruland’s disc couplings feature two anodized aluminum hubs bolted to one or two stainless steel disc packs (also called disc springs). They are designed for zero-backlash operation and offer an excellent middle ground of high torsional rigidity with moderate misalignment capability.

There are two common configurations:

• Single Disc Coupling: Consists of two hubs connected by a single flat disc pack. This design is very torsionally stiff and can handle a few degrees of angular misalignment plus a small amount of axial motion. However, a single disc cannot compensate for much parallel misalignment (offset) – the disc would have to bend laterally, which it can only do minimally.
  
• Double Disc Coupling: This adds a center spacer between two disc packs, essentially creating two flex points. One disc pack flexes one way and the other flexes the opposite way, enabling the assembly to accommodate parallel misalignment in addition to angular and axial misalignment. The double disc design preserves the coupling’s overall torsional stiffness while greatly increasing misalignment capability. Most servo applications requiring disc couplings will use the double disc style for this reason.
  

Disc couplings are all-metal and have no internal damping, similar to bellows couplings. They excel at maintaining constant velocity and precise control. The thin discs flex in plane, so bearing loads due to misalignment are kept relatively low. Torsionally, disc couplings are extremely rigid, just slightly less stiff than an equivalent bellows coupling, which is often a worthwhile trade-off for the added misalignment allowance. Ruland’s disc couplings are balanced for high speeds (commonly used up to ~6,000 RPM or higher), and the aluminum hubs keep the inertia low.

Ruland manufactures disc couplings in a variety of sizes (commonly up to 1-1/4″ or 32 mm bore) and in both inch and metric hub combinations. Clamp-style hubs ensure a secure, concentric hold on the shafts (keyway options are available by suffix “K”). A typical part number is DCD21-8-8-A, which specifies a double disc coupling (DCD) of size 21 (1.313″ OD) with two 0.5″ (8/16) bores, aluminum hubs. The single disc versions use a “DCS” prefix. An electrically isolating version is also offered (using a non-metallic center spacer or insulating bushings) if you need to prevent metal-to-metal contact between shafts.

Controlflex Couplings

Ruland also supplies a specialized disc-type coupling called the Controlflex. Controlflex couplings use a unique “frog-leg” shaped plastic insert instead of metal disc springs. They are a three-piece assembly with two aluminum hubs and either one insert (single Controlflex) or two inserts (double Controlflex) joining them. The curved plastic element allows large misalignments (several degrees of angular and significant parallel offset) with ultra-low bearing loads, as the flexing limbs of the insert take up misalignment very gently. Controlflex couplings have zero backlash and are electrically isolating (the metal hubs do not touch, and the insert is acetal). They are specifically designed for sensitive instruments like encoders, resolvers, or tachometers, where even tiny side loads or vibrations must be minimized.

Because of their polymer element, Controlflex couplings are not for very high torque – they serve small-to-medium motion systems that value misalignment accommodation over torsional rigidity. The benefit is that they protect delicate components (small motor bearings, glass encoders, etc.) from stress. For example, a single-insert Controlflex might handle ±1° angular and 0.25 mm parallel misalignment with negligible bearing load. Ruland’s Controlflex hubs are sold separately (e.g., part CPRS19-6-A for a single-insert hub with 6 mm bore, or CPRD30-10-A for a double-insert hub) and are combined with the matching center “frog” insert to build a complete coupling. These couplings come in bore sizes from 3 mm to 40 mm, and both clamp and keyed versions are available.

Jaw Couplings (Curved Jaw)

Jaw couplings are a popular servo coupling option known for their robust performance and vibration damping. Ruland’s zero-backlash jaw couplings consist of two metallic hubs (aluminum or stainless steel) with curved jaws that interlock, and an elastomeric spider insert sandwiched between the hubs. The spider’s lobes are pre-compressed between the jaws, eliminating any free play (backlash) under normal operation. This curved jaw design ensures constant contact for torque transmission even as the coupling flexes, and it maintains zero backlash as long as the spider is intact.

Key features of jaw couplings:

• Multiple Spider Durometers: Ruland offers spiders in three hardness levels (measured in Shore A durometer) to tailor the coupling’s performance. 85 Shore A (Blue) spiders are softest – they provide the greatest vibration damping and shock absorption, which protects the system, but at the cost of lower torsional stiffness and torque capacity. 92 Shore A (Yellow) is a medium hardness for balanced performance. 98 Shore A (Red) spiders are the hardest – they offer the highest torsional rigidity and torque capability, but minimal damping. All spiders are polyurethane and electrically isolate the two hubs (no metal-to-metal contact).
  
• Misalignment and Shock Absorption: Curved jaw couplings can handle moderate misalignment (about 1° angular and a small amount of parallel offset, typically a few thousandths of an inch). They excel at cushioning shock loads and damping vibration in servo systems – for example, if a motor rapidly starts/stops or reverses, the spider element absorbs some of the shock, reducing wear on other components. This makes jaw couplings ideal for servo-driven systems with stop-and-go motion or those prone to torsional oscillations.
  
• Fail-Safe Design: If the spider insert were to wear out or shear under extreme overload, the metal jaws of the hubs will come into contact and continue to transmit torque. This metal-on-metal drive is not precise (backlash will be present, and the coupling will stiffen), but it can keep the system operational to prevent a sudden shutdown. The curved jaws ensure that even in this state, the hubs can still interlock and drive the load in an emergency.
  

Ruland’s jaw couplings are designed for balanced high-speed operation (up to 8,000 RPM) and zero-backlash performance. They are available in clamp or set screw hub styles, in aluminum for general use or stainless steel for higher corrosion resistance. Users can mix and match hubs with different bore sizes, and even mix inch and metric bores in one coupling by selecting the appropriate hubs. For example, one could use hub MJC41-15-A (metric clamp style, size 41 coupling, 15 mm bore) with hub JC41-8-A (inch clamp style, size 41 coupling, 1/2″ bore) plus a spider to build a hybrid coupling for a 15 mm to 0.5″ shaft connection. (The size “41” corresponds to the coupling’s OD of 1.63″ in this case.)

When selecting a jaw coupling, choose the largest coupling size that fits your shafts (larger size = higher torque capacity and misalignment capability) and then select the spider hardness based on the desired trade-off between rigidity and damping. Our Jaw Coupling category page (link to Jaw Coupling category page) provides details on available Ruland jaw coupling hubs and spiders. These couplings are an excellent choice for servomotor applications that can tolerate a bit of compliance in exchange for shock protection and flexibility, for instance, in packaging machinery, indexing conveyors, or any system where moderate precision is sufficient but reliability and longevity are paramount.

Oldham couplings are three-piece couplings consisting of two hubs (usually aluminum or stainless) and a center disk insert made of engineered plastic. They are renowned for handling larger parallel misalignments than other coupling types, while operating with zero backlash and providing a mechanical fail-safe. In a Ruland Oldham coupling, each hub has a drive tenon (tab) that fits into a slot on the center disk – one hub’s tenon engages on one face of the disk, and the other hub’s tenon engages the opposite face, at 90° offset. Torque is transmitted through the disk via these tongue-and-groove engagements.

Key characteristics of Ruland Oldham couplings:

• High Parallel Misalignment Capability: Oldham couplings can typically accommodate up to 5% – 10% of their outer diameter in parallel offset. For example, a 1″ (25 mm) OD Oldham coupling can allow around 0.050″ (1.3 mm) of lateral misalignment between shafts – far more than other servo coupling types. This makes Oldhams extremely useful when precise shaft alignment is difficult to achieve or when shafts experience lateral movement.
  
• Zero Backlash (with Wear Consideration): The disk is a slight press-fit in the hub slots, eliminating backlash when new. Over time, the sliding action will wear the plastic disk and eventually introduce a small amount of backlash. Fortunately, the fix is simple: the disks are inexpensive, replaceable items, and installing a new disk restores the coupling to zero-backlash performance. In practice, the maintenance interval for disk replacement will depend on the application’s speed, misalignment, and duty cycle, but periodic inspection is recommended in high-precision systems.
  
• Consistent Low Bearing Loads: Because misalignment is accommodated by the disk sliding between the hubs, the reaction force does not substantially increase as misalignment grows (unlike beam or bellows coupling, where greater bending leads to higher forces). Oldham couplings impose very low and steady bearing loads, which is great for protecting motor and driven shaft bearings. Even at maximum offset, the friction is low – one reason Oldhams are popular in applications like conveyors and linear systems.
  
• Torsional Properties and Damping: An Oldham coupling’s torsional stiffness is moderate. There is some wind-up due to the plastic element, though an acetal disk insert is fairly rigid and yields good positional accuracy for most uses. If more shock absorption is desired, a softer disk material like nylon can be used at the expense of some rigidity. Ruland offers disk inserts in several materials – typically acetal (AT) for the highest torque and stiffness, nylon (NL) for better vibration damping and shock absorption, and PEEK for high temperature or special chemical environments. The plastic disk also inherently dampens noise and vibration to a degree, especially with the softer nylon option.
  
• Electrical Isolation and Fail-Safe: With a non-metallic center disk, Oldham couplings electrically isolate the two shafts, which can be important for preventing ground loops or protecting sensitive components. Additionally, the disk can act as a mechanical fuse: if the system torque exceeds the coupling’s rating or if there is a jam, the disk will shear and disconnect the drive, protecting more expensive equipment down the line. In such an event, the disk typically breaks cleanly and can be quickly replaced to restore operation.
  

Selecting an Oldham coupling involves choosing hub styles and a disk material. Ruland makes this easy with a modular system: hubs are available in clamp (prefix OC or OCT) or set screw (OS/OST) designs, with bore sizes from about 1/8″ up to 1-1/8″ (3 mm to 30 mm). You can combine any two hubs and one disk to create a complete coupling. For example, a complete assembly might use two hubs, OCT25-10-A (clamp style, 1.00″ OD, 10 mm bore, aluminum) and a disk, OD25-AC (1.00″ OD disk, acetal). Mixing inch and metric bores or clamp and set-screw hubs is perfectly fine – one reason Oldhams are very popular in custom machinery. Stainless steel hubs are available for washdown or vacuum environments (they pair with the same disks).

For sizing, choose an Oldham coupling with an OD large enough to handle your torque and misalignment (larger OD = higher torque capacity and more misalignment allowance). Check the torque rating with the chosen disk material (acetal disks have higher torque capacity than nylon, etc.). Our team can assist with specifying the correct part numbers – see Ruland Oldham Couplings (link to Ruland Oldham Couplings) for more details on options and to get in touch.

The table below provides a quick comparison of these Ruland servo coupling types and their performance characteristics:

Conclusion

Choosing the right Ruland servo coupling comes down to balancing precision, misalignment, and durability requirements. In summary, bellows couplings provide the highest precision (zero backlash and maximal rigidity) for well-aligned high-speed systems. Disc couplings closely follow, adding a bit more misalignment tolerance with minimal loss in stiffness. Beam couplings are an economical all-rounder for moderate torques and misalignments, offering zero backlash and maintenance-free simplicity. Jaw couplings and Oldham couplings introduce polymer elements that allow damping and larger misalignments – jaw couplings excel in absorbing shock and are fail-safe, while Oldham couplings handle significant parallel offset and are easy to service by swapping inserts. For delicate instrumentation or extreme misalignment with minimal torque, Controlflex couplings fill the niche by safeguarding small bearings and encoders.

PIB Sales is proud to be an authorized distributor of Ruland Manufacturing, supplying the full range of these servo couplings. Engineers and purchasing teams can rely on PIB Sales for accurate technical guidance, genuine Ruland parts, and fast delivery. You can conveniently order standard Ruland couplings directly from our online store, or contact our team for custom needs (special bore sizes, keyways, etc.).

Purchase Options – How to Order:

• Find and order Ruland servo couplings on our website (quick search by part number or coupling type).
  
• Call us at 1-800-228-8895 to speak for advice or to place an order.
• Send your inquiry or RFQ to [email protected], and our team will promptly assist with pricing, availability, and part selection.

Not sure which coupling you need? Reach out for free engineering support – we’ll help verify the sizing, recommend the right coupling type, and provide any data you require.

With PIB Sales, you get a trusted partner to ensure you select the optimal Ruland coupling for your servo application. We make the purchasing process easy, from online ordering of standard catalog parts to one-on-one application support for unique requirements.

Ready to get started? Explore our catalog of Ruland servo couplings online or contact us today to discuss your needs. We look forward to helping you improve your motion control system with high-quality Ruland couplings.

FAQ

Q: Are all Ruland servo couplings truly zero-backlash?
A: Yes. All of the coupling styles discussed (beam, bellows, disc, jaw, Oldham, Controlflex, and even rigid couplings) are designed for zero backlash performance. This means they transmit motion without any free play. However, keep in mind that wear or heavy loading can introduce a tiny amount of backlash over time in couplings that have replaceable inserts (jaw and Oldham). Regular maintenance (inspecting and replacing worn spiders or disks) will ensure they remain backlash-free.

Q: Which coupling type can handle the most misalignment?
A: Oldham and Controlflex couplings handle misalignment best. Oldham couplings excel at parallel misalignment (up to 10% of the coupling diameter as offset). Controlflex couplings can accommodate large misalignments in all directions (angular, parallel, axial) with minimal bearing stress, making them great for small, delicate systems. Beam couplings can handle significant angular and axial misalignment as well, especially the six-beam designs, but less parallel offset than Oldham’s. Bellows and single-disc couplings have the least misalignment capability and should be used in well-aligned setups.

Q: How do I choose the right spider hardness for a jaw coupling?
A: Choose the spider durometer based on the needs of your application. If you require maximum precision and torque, go with the harder 98 Shore A (red) spider – it minimizes wind-up (more torsional stiffness) but provides the least shock absorption. For general purpose or if moderate damping is beneficial, the 92 Shore A (yellow) is a balanced choice. If your system experiences frequent shock loads or you want to protect fragile components from vibration, the 85 Shore A (blue) spider is best – it will soften impacts the most (at the cost of some positional accuracy). Remember that all three spiders will operate zero-backlash in a Ruland curved jaw coupling due to their preload.

Q: Do Ruland couplings require any maintenance?
A: Most Ruland couplings are maintenance-free. Beam, bellows, disc, and Controlflex couplings are all-metal or one-piece designs that do not wear out under normal use – just install and they run without attention. Jaw and Oldham couplings have polymer components (spiders and disks) that can wear over the years of service or under harsh conditions. It’s wise to inspect those inserts periodically. Replacing a worn spider or disk is quick and inexpensive, and restores the coupling’s original zero-backlash performance. PIB Sales can supply spare spiders and disks for maintenance needs.

Q: Can I mix inch and metric bore sizes in one coupling?
A: Yes – many Ruland coupling designs allow mix-and-match of hubs to suit different shaft sizes. For jaw, Oldham, disc, and Controlflex couplings, you can specify each hub with an inch or metric bore as needed (for example, one side 1/2″ and the other side 12 mm). Ruland’s part number system makes it easy to select each hub with the desired bore and style. The only limitation is within one-piece couplings like beam or bellows types: those are bored straight through, so both shafts must be the same series (either both inch or both metric diameters, although Ruland does offer some “inch-to-metric” sizes in bellows and disc couplings). If you have a mixed-dimension shaft pairing, a two-hub coupling style (jaw, Oldham, disc, Controlflex) will be the solution. Feel free to contact PIB Sales for assistance in configuring the right combination of hubs and inserts for mixed units.