Machine tools are important types of machines, used throughout the mechanical engineering field. Most can be categorised into one or more of milling machines, drills, lathes, grinders, saws, shaping machines, and gear cutting machines. Another grouping contains erosive machine tools, such as laser, water jet, wire and sinker EDM (electronic discharge machining). New technologies, enabling higher process speeds, greater machine stiffness, etc. constantly increase the demands placed on these machines and their components.
Connecting elements in the machine tool
A variety of individual components can be found across various machine tool technologies. In addition to drives, linear guides, tool holders and workpiece fixtures, these common component parts include connecting elements such as zero-backlash couplings. Special attention should be paid here to the differing influences that zero-backlash metal bellows couplings and zero-backlash elastomer couplings have on a machine. These connecting elements enable the machine to achieve maximum stiffness (metal bellows coupling) or vibration-damping (elastomer or jaw coupling), the requirements for which vary depending on the application. Following is an overview of the important influencing factors of these two different connecting elements.
Modern metal bellows couplings (Fig. 2) generally consist of two parts: a stainless steel bellows and various types of end hubs. Depending on the application and the associated geometry, hubs are designed as flanges, clamping hubs, expanding shafts or conical clamping sets. The hubs are typically made of aluminium, carbon steel or stainless steel. Nickel plating, chrome plating, nitrocarburizing, anodizing and black oxiding are often applied to ensure that the surface of the hub is protected efficiently. The hubs serve to connect the coupling to the driving and driven components of the machine. Positioned between the drive hub and driven hub, the stainless steel bellows, which is usually double-walled, compensates for incidental misalignment between the driving shaft and the driven shaft. This compensatory function plays an important role in the driveline as a whole. Metal bellows couplings with high torsional stiffness are typically used in machine tool drives with torque ranges from 0.1 Nm to 10,000 Nm and bore diameters ranging from 3 mm to 180 mm.
By definition, torsionally stiff, flexible couplings should exhibit the highest possible stiffness in the direction of rotation. The spring stiffness in the axial, angular and lateral directions should also be as low as possible, so that the driveline is not strained by transverse forces resulting from misalignment. Metal bellows couplings have very high torsional stiffness in the direction of rotation. This means that the coupling undergoes only minimal twisting distortion under a torque load. As a rule, the maximum torsion angle should be between 0.05 – 0.1 degrees, though it is possible through sizing to achieve even greater precision. This characteristic enables the transmission of rotational movements from the driving component to the driven component with the least possible positioning error.
Elastomer materials play an important role in many areas of mechanical engineering today, as a result of their high strain capacity combined with resilience and damping characteristics. In drive technology, elastomer couplings are primarily used to provide shock absorption and torsional vibration damping in situations where impact loads and vibration are expected.
Elastomer couplings or jaw couplings (Fig. 3) consist of two components: the elastomer insert and various hub designs. As with metal bellows couplings, the hubs are needed in order to connect the driving components to the driven components. Depending on the mechanical interface of the components to be connected, the connection can be accomplished with clamping hubs, split clamping hubs, conical clamping sets or keyways. Due to its design and material properties, the elastomer insert, also known as a spider, is capable of providing a damping effect, while also facilitating torque transmission and compensating for shaft misalignment. The torque ranges covered by zero-backlash elastomer couplings typically span from 2 Nm to 25,000 Nm. The bore diameters range from 3 mm to 170 mm.
Vibration-damping elastomer couplings filter vibration and shock out of the machine drive. The elastomer insert reduces and compensates for torque shocks through its elasticity and resilience. It also absorbs vibration between the driving and driven components, resulting in the machine axis as a whole running more smoothly.
Summary table for both coupling systems
| Properties | Metal bellows couplings with high torsional stiffness | Vibration-damping
Elastomer couplings |
| Precise transmission | ++ | + |
| High torsional stiffness | ++ | 0 |
| Low restoring forces from misalignment | ++ | + |
| Misalignment compensation | ++ | + |
| Vibration damping | 0 | ++ |
| Ease of installation | + | ++ |
| Low moment of inertia | ++ | ++ |
| Compact dimensions | + | ++ |
| Smooth operation (application dependent) | ++ | ++ |
The summary table shows as clearly as possible the general differences between the two coupling systems. While individual selection can be more complex, the crucial question in the relevant area of the machine tool is:
“Does this area require precise transmission with a minimum torsion angle or is it more important that vibration and shock are removed by the connecting element? ”
A uniform “one size fits all” solution that suits each individual manufacturer and machine has yet to be found. Both coupling systems are primarily used for connections between the servo or stepper motor and either the linear motion system or the cutting tool spindle. Depending on the machine controls, acceleration rate, inertia ratio, feed screw pitch, etc., an exceptionally stiff transmission (metal bellows coupling) or a slight damping effect (elastomer coupling) must be decided upon by engineers and / or recommended by component manufacturers for optimal machine dynamics.
The differences between the two coupling systems and their properties clearly show that attention must be paid to choosing the right coupling when building new machine tools. Assistance is offered by most reputable coupling manufacturers, whose experience and competent advice on each application can help to ensure a reliable solution. R+W Antriebselemente GmbH offers the machine tool market assistance in finding the right solution for their required specifications. Large and well known, as well as small specialty machine tool builders alike commonly turn to this lower Main valley coupling expert for compact, high performance coupling solutions.
Article Copyright R+W, July 2018