Rigid Couplings Guide

Rigid couplings – sometimes called rigid shaft couplings or sleeve couplings – are solid shaft connectors that join two shafts without allowing any relative motion. In high-precision motion control systems (e.g., servo-driven equipment), rigid couplings provide exceptional torsional stiffness and zero backlash performance. This means there is virtually no wind-up under torque and instantaneous power transmission, making them ideal for accurate positioning in servo coupling applications. However, unlike flexible couplings, rigid couplings cannot accommodate misalignment or absorb vibration. Any angular or parallel misalignment is directly transmitted, which can stress shafts and bearings if not properly controlled. Thus, rigid couplings are recommended only when precise alignment can be ensured. When used correctly, they provide a simple, robust, and maintenance-free method for connecting shafts in production machinery and motion control systems.

In this article, we explore the main types of clamp-style rigid couplings – one-piece, two-piece, and three-piece designs – along with specialized variants like step bore couplings and double wide shaft collars (used as compact couplings). We compare their characteristics (torsional rigidity, strength, backlash, etc.) in a side-by-side table, and provide design insights and installation tips. Finally, we outline how to select and order high-quality rigid couplings from PIB Sales, an authorized Ruland distributor.

Clamp-Style Rigid Coupling Types

All the rigid coupling designs discussed here are clamp-style couplings. Clamp-style rigid couplings wrap around the shaft and use tightening screws to secure the connection, rather than driving a screw point into the shaft as with older set-screw couplings. This design provides high holding power without marring the shaft surface and ensures a zero-backlash fit. Below, we describe the key types of clamp-style rigid couplings: one-piece, two-piece, and three-piece.

One-Piece Rigid Couplings

One-piece rigid couplings are made as a single integrated collar (usually with one or two factory-cut slots to allow tightening). This solid one-piece design offers excellent inherent balance and high torsional strength. A one-piece clamp coupling must be slid into place over the shaft ends during assembly, so it requires that at least one shaft is free to move. While extremely robust, the one-piece style is less convenient if frequent disassembly is needed (since you would have to slide off components to remove it). In practice, one-piece rigid couplings can be evaluated for applications up to around 3000 RPM when precisely machined and properly installed. They are ideal for simple, aligned shaft connections where a compact and strong zero-backlash coupling is required.

Two-Piece Rigid Couplings

Two-piece rigid couplings consist of two halves that clamp together around the shafts. Each half has an identical bore, and they bolt together via cap screws. The two-piece design enables installation and removal without requiring the movement of other components or shaft assemblies – a significant advantage for maintenance. Because the two-piece coupling can be assembled in place, it’s often used in equipment where shafts are fixed in position. Suppose the hardware (screws) on a two-piece rigid coupling is arranged in opposing positions (180° apart). In that case, the assembly is dynamically balanced and can operate at higher speeds, roughly up to 4000 RPM. Two-piece clamp couplings provide the same zero-backlash, high-torque performance as one-piece versions, while being easier to work with when servicing machinery.

Three-Piece Rigid Couplings

Three-piece rigid couplings (sometimes called double-split couplings) incorporate two clamping end sections and a removable center spool. This design is less common but very useful for certain situations. A three-piece coupling allows one to disconnect one shaft without disturbing the other: either end section can be detached from the center piece, letting you remove one shaft independently. This makes maintenance and machine servicing much easier, especially if one of the shafts (for example, a motor shaft) cannot be moved axially. Additionally, the three-piece rigid coupling clamps each shaft independently, which can accommodate a slight difference in shaft sizes by effectively gripping each shaft on its own. In other words, the coupling can handle minor diameter variation or fit issues better than a solid one-piece coupling. Three-piece rigid couplings are designed for easy maintenance and exchange of components while still providing the same torsional rigidity and zero backlash as other clamp types.

Installation and Clamping Tips: For all clamp-style rigid couplings, consider these best practices during installation and design:

• Use of Cross-Cuts: Many rigid couplings (especially two-piece and three-piece styles) include an additional cut or cross-slot through the body. When combined with pairs of screws placed close together, this cross-cut feature allows the coupling to flex just enough to grip the shaft more uniformly. It facilitates greater holding power and can accommodate slight deviations in the shaft diameters.
  
• Alternate Screw Tightening: Always tighten the screws in small increments and in an alternating pattern, rather than fully tightening one screw at a time. For couplings with two screws adjacent to each other on a side, tightening one can relax the other’s tension. By alternating across each pair of screws in steps, you distribute clamping stress evenly and achieve maximum holding power.
  
• Anti-Vibration Hardware: Rigid couplings in motion systems are subject to vibration, which can loosen hardware over time. To combat this, high-quality manufacturers like Ruland apply a nylon patch to the threads of coupling screws. This Nypatch coating adds friction to prevent screws from backing out and also prevents galling of threads (especially in stainless steel couplings). Using couplings with such vibration-resistant hardware increases reliability in the long run.

Step Bore Rigid Couplings

Not all shafts in a system have the same diameter, and that’s where step bore rigid couplings come in. A step bore rigid coupling is a coupling with two different-sized bores (one on each end), allowing it to connect shafts of unequal diameters directly. For example, you might use a step bore coupling to join a 10 mm motor shaft to a 3/8″ lead screw without any additional adapters. This design maintains a solid, one-piece connection between the dissimilar shafts, eliminating the need for bushings or sleeve adapters. Each bore is precision-machined to its specified diameter – often precision honed for perfect fit and alignment – so that both shafts are held collinear and secure. The result is a zero-backlash, high-rigidity coupling that can bridge metric and imperial shaft sizes (or any two different diameters) in one compact component. Engineers often choose step bore couplings in servo and motion control systems that involve mixed shaft sizes or retrofits, as they simplify the drivetrain design while preserving alignment and torsional stiffness. This is a very useful and practical solution where you have the need to accommodate a design with two different-sized shafts.

Double Wide Shaft Collars as Compact Couplings

An alternative to a traditional rigid coupling for tight spaces is the double-wide shaft collar used as a coupling. A double-wide shaft collar is essentially an extra-wide one-piece collar that spans across two shaft ends, clamping down on each shaft to hold them together. This solution is useful in applications where space is very limited and a standard coupling’s length or diameter would be too large. The trade-off for the collar’s compact size is a somewhat reduced torque capacity. Because a double wide collar typically uses only a single cap screw on each side (one per shaft) rather than two or more screws, it cannot transmit as much torque as a comparably sized multi-screw rigid coupling. However, the smaller mass and shorter length of the collar mean lower rotational inertia, which is beneficial for high-speed or quick-reversing motion control applications. In practice, the double-wide shaft collar provides a rigid, zero-backlash connection ideal for light-to-medium-torque needs. It’s often chosen in servo systems, robotics, or packaging machines where minimizing weight and size is critical, and where the available torque from the motor can be handled with a single-screw-per-shaft design.

Comparison of Rigid Coupling Types

The table below compares key characteristics of the various rigid coupling options discussed above to aid in selection. It highlights torsional rigidity, relative torque capacity, backlash, and notable features of each type:

Note: All of the above rigid couplings provide essentially zero backlash and extremely high torsional stiffness. The best choice depends on your application’s specific needs – for example, whether quick assembly/disassembly (two-piece or three-piece), space constraints (double wide collar), or different shaft sizes (step bore) are primary factors, alongside the required torque and speed. In critical servo systems, ensuring precise shaft alignment and using quality couplings with proper installation will maximize performance for any of these types.

In conclusion, as an authorized distributor of Ruland Manufacturing, Pacific International Bearing Sales Inc (PIB) offers easy online ordering for Ruland rigid couplings. 

Place your order for Ruland rigid couplings online through the PIB Sales website at pibsales.com.

 For further assistance or to place an order with personal support, send an email to [email protected] or Call  (800) 228-8895.