What is a Keyed Joint? | Shaft-Hub Connection | Calculation | Method C | Design

In this video we take a closer look at the keyed joint. The keyed joint is one of the shaft hub connections also known as positive locking connections. The keyed joint is used for example in gearboxes to connect gears with the gearbox shaft and to transmit torques. The hub is the component that is to be connected to the shaft in a rotationally fixed manner. This can be for example a gear a belt pulley a coupling disc a flywheel a fan wheel or a brake disc.

The force transmission between the shaft and the hub takes place through the key. It is inserted into an axial groove that is milled into the shaft (keyseat) and protrudes a few millimeters from the shaft. In the gear hub there is also an axial groove (keyway) so that the gear can be pushed onto the shaft and the key. The force can now be transmitted in a form fitting manner from the shaft through the key to the hub.

Since the keyed joint is a detachable connection the components can be easily assembled and disassembled. The axial movability can also be intentionally desired for example in a sliding clutch. In this case it is referred to as a sliding fit and the key is called a sliding key or a floating key.

However a disadvantage can be the reduction of the shaft cross section caused by the milled grooves. In addition the grooves in the hub and the shaft cause stress concentrations that lead to increased peak stresses. Furthermore unlike a pure interference fit the keyed joint must be secured in the axial direction if axial movement is not desired which increases the design effort due to the use of a retaining ring or a lock nut.

In addition tolerances between the key the shaft and the hub can cause slight clearance. Under varying loads over time this can lead to wear of the grooves. An exact positive locking of the hub by the shaft is then no longer possible. Furthermore microscopic relative movement between the shaft and the hub over time causes so-called fretting corrosion. This also leads to material weakening over time.

For the design of keyed joints DIN 6892 distinguishes a total of three methods. Method A is the most complex and deals with the strength verification of the shaft the key and the hub under real conditions through a realistic experiment or through extensive numerical calculation methods such as the finite element method. The standard does not specify a concrete procedure for calculation method A.

Calculation method B is less complex and the procedure is standardized. Method B specifically takes into account the surface pressure between the key and the groove in the hub as well as the groove in the shaft. This procedure is based on the assumption that the main failure criterion for the keyed joint is the permissible surface pressure between the shaft keyseat and the key or between the key and the hub keyway. Nevertheless the strength verification for the shaft must be carried out according to the nominal stress concept when using method B. Method B also considers impact loads load reversals load peak frequencies friction factors inhomogeneous load distribution along the length of the key as well as support and hardness influence factors.

For the preliminary design of keyed joints there is also calculation method C. This method is also based on the assumption that surface pressure is the critical failure criterion of the keyed joint. For simplification a constant surface pressure over the entire length and height of the key is assumed.

Calculation method C can only be applied if the direction of the torque does not change. Also if the length of the key exceeds 1.3 times the shaft diameter method C must not be used because the shaft the key and the hub then deform too much and the surface pressure is no longer constant over the entire length of the key.

00:00 Shaft-Hub Connection
00:42 How Does a Keyed Joint Work?
01:36 Advantages of Keyed Joints
02:16 Disadvantages of Keyed Joints
03:17 Tight Fit of the Key
04:15 Loose Fit of the Key
04:51 Sliding Fit of the Key (Floating Fit)
05:43 Design Consideration: Stepped Hub
07:07 Calculation Methods A, B, and C
09:10 Power Transmission
10:21 Determination of the Contact Force from the Torque
11:48 Determination of the load-bearing height of the contact surface
13:19 Determination of the Load-Bearing Length of the Contact Area
14:28 Calculation of the contact pressure
15:45 Example: Slotted Locknut
17:18 Example for calculating the required key
20:04 Use of Two Keys (Load Factor)