How shaft-hub connections have evolved

Today three types of shaft-hub connection are commonly used by machine builders, namely keyways, mechanical connections and hydraulic clamps. We will consider each type in turn.

Keyway connections

A keyway connection consists of a slot cut in a shaft component, receiving part of a key that also engages with an equivalent slot on a mating hub component, thereby locking the two parts together.

While keyways are cheap to produce and well standardised, disadvantages include:

• Disadvantageous for mounting, dismounting and adjusting
• As forces are concentrated around a small area, bigger shafts are needed to transmit torque{
• ] which results in over-dimensioning
• Increased inbalance and weaker precision of the mounted shaft and components
• Prolonged, meticulous assembly required, as the exact location of the keyway needs to be established
• Keyways weaken shafts, causing an increased risk of fretting, fatigue and breakage

Mechanical connections

Mechanical clamping connections consist of two sleeves with tapered surfaces that are pressed against each other along the shaft. Many screws with a high tightening torque are used to hold it together. When mounted the two sleeves deform into contact with the bore of the hub and the surface of the shaft. This technique is used in rough applications where surfaces finish and tolerances are variable.

While this standard connection is suitable for transmitting high radial loads, bending torque and can withstand temperature variations, disadvantages include:

• Sleeves move axially, making hub positioning difficult
• High tightening torque required on screws to overcome internal friction forces on tapered sleeves
• Difficult to get good runout; screws must be tightened evenly and in a certain pattern
• Special dismantling screws are needed to overcome internal friction between sleeves and to release the connection
• Disadvantageous for mounting, dismounting and adjustment

Hydraulic clamping connections

Hydraulic clamping connections enable effortless mounting, adjustment, easy reach and a well-balanced design. The technique is a problem-solver for critical applications. ETP hydraulic clamping connections are beneficial for working with tight tolerances, resulting in superior runout.

French scientist Blaise Pascal devised the maxim of pressure propagation in liquids during the 17th century. The principle refers to pressure changes in a liquid within a confined container where pressure transmits evenly throughout the liquid. When applying pressure at one point of the liquid, an even pressure change occurs throughout the whole container (see below).

ETP hub-shaft connections are guided by Pascal's principle. The products consist of hardened double-walled steel sleeves that are welded together to create a hollow chamber; this chamber is filled with a hydraulic pressure medium.

How a hydraulic clamping connection works

By the low force of your hand tightening the screw, an internal pressure of approximately 1000bar is created. This causes the sleeves to expand evenly in all directions around the hub and shaft. The result is an even surface pressure and a secure and precise connection between the hub and shaft.

By using the ETP principle, a hydraulic shaft-hub connection offers a number of advantages compared to the alternatives:

• Easy and accurate positioning along the shaft, as no axial movement occurs during tightening
• Mounting and dismounting takes seconds
• Excellent runout as the connection self-centres
• Compact, with small built-in dimensions
• Free space along the shaft as there is no need for dismantling screws or special tools

Quick, easy, precise

ETP is said to be the only company manufacturing a standard range of hydraulic hub-shaft connections. These hydraulic connections create an even surface pressure around the hub and shaft. Mounting and dismounting of ETP connections takes only a matter of seconds. Furthermore, ETP hydraulic connections achieve excellent runout because they are self-centring.

Available through Abssac, this is a truly innovative product. All ETP products consist of a double-walled hardened steel (in some cases stainless steel) sleeve, filled with a pressure medium. Within the connection flange there are one or more screws and a piston with seals for the pressure setting. When the pressure screw is tightened an even and moderately high surface pressure is created against the shaft and hub, causing the locking effect. The self-contained product allows the clamping procedure to be repeated many times. Whether the hubs are to be removed or repositioned, mounting and dismantling takes only a few seconds. The hydraulic principle gives immediate advantages for today's machine builders.

Follow the link for more information about ETP hydraulic shaft-hub locking bushes from Abssac.