Parallel keys
(67)A safety key is usually a metallic element used to connect a rotating machine component with a shaft. It is commonly used to prevent relative rotations between two parts and may also be applied for torque transfer purposes. For a shaft key to function correctly, a rotating component (such as a pulley or gear wheel) and a shaft should come with a seat and a keyway. A pre-cut hole is located in the shaft, and another similar groove is located in the hub, and it is into this space that the securing elements are usually inserted to connect the two parts permanently. The whole system is described as a keyway connection and is able to provide relative axial movement between two parts.
To sum up, a key and a keyway form a keyway connection, which secures the hub and shaft by preventing relative movement between the power transmission shaft and the attached component. For example, gear drives, pulleys or sprockets are securely connected with a shaft by means of a key. There are five major types of keys, i.e. flush key, saddle key, tangential key, round key and splined key.
Safety keys – shapes and materials
These elements fit into keyways of both the shaft and the secured component in the machine. A countersunk piece of steel is recessed into the shaft down to half its thickness, where the measurement is taken from the side of the key.
There are several most commonly used types of keys, including:
- rectangular keys – most commonly cuboid-shaped pieces of metal;
- rounded keys – cuboid-shaped pieces with rounded ends;
- parallel keys with an opening facilitating additional attachment of elements using screws to reinforce the joint.
Each key type can be further categorised and, for example, keys with an opening can be: chamfered single-hole keys, chamfered double-hole keys, rounded single-hole keys, etc.
These elements can be made of different materials. Generally, they are manufactured from carbon steel or stainless steel, but they can also be made of brass, copper, aluminium, monel and even nylon, however, with high availability of steel and its properties, it is the most commonly used material to manufacture keys. AISI 316 steel can also be used for marine applications
Standardisation systems specifying properties of keys – DIN 6885 standard, etc.
In Poland, dimensions of parallel keys and their keyways are specified in two standards:
- PN-70/M-85005
- DIN 6885
As per the applicable Polish standard, a key is designated with its type and dimensions: width (b), height (h) and length (l), separated by a symbol “x”.
There is also a British standard available, i.e. BS 4235: Part 1: 1972 (1986), specifying square and rectangular parallel keys and keyways, etc.
Three classes of fit are provided for square and rectangular parallel keys and keyways, namely “free”, “normal” and “closed”. The “free” class of fit is used when a given application requires an assembly hub to move over the key; the “normal” class of fit is used when a key is to be inserted into a keyway with minimum fitting involved, as may be required in mass-production assembly operations; the “closed” class of fit is used when a key must be fitted precisely in relation to maximum material-related conditions that may have to be followed while selecting components.
The standard does not take into account misalignment or offset greater than that which can be accommodated in the dimensional tolerances. If an assembly operates in heavy-load conditions, it should be checked whether the cumulative impact of misalignment or offset, or both, does not prevent satisfactory key seating. Unless specified otherwise, keys conforming to this standard are made of steel specified in the BS 970 standard, with tensile strength amounting to 550 Mn/m2 in a ready-made condition.
Correct shaft key selection is critical to avoid premature failure of keyway connections. Before selecting a key, it is worth familiarising yourself with detailed rules on shaft key selection and shaft keyway designing, in order to understand how to calculate both shearing and compressive stresses affecting the shaft key. There are numerous critical factors, such as materials, type of load, correct fit, etc., that need to be considered when designing and constructing a keyway connection in order for these safety features to do their job. The dimension tables included in applicable standards facilitate the selection of correct parameters for these components.
Parallel keys vs. parallel taper keys and inch parallel keys
A parallel key is an element of a keyway connection used to connect a hub with a shaft. These are the most common types of connections for such components used in both cylindrical and tapered joints. They are made of steel with minimum tensile strength. There are various pros and cons of using keys, so attention must be paid to the finer details of the overall design to assess the suitability of a particular keyway connection.
As per most standards, the dimensions related to key are specified in millimetres, but there are also those whose size is given in inches. When buying such a product, it is always a good idea to pay attention to the units on which the key size is specified.
Keys without a taper are simply called “keys”, while keys with a taper are also sometimes referred to as “taper keys”.
Advantages of a keyway connection
- Low manufacturing costs and high availability;
- Wide range of specifications provided in standards (ISO, BS, DIN and ANSI);
- Torque transfer – from medium to high values;
- Easy installation and disassembly, which facilitates their reuse
Disadvantages of a keyway connection
- Cannot be used to support variable directional loads and shocks;
- Possible hub axial displacement, unless an additional locking element (e.g. a set screw or retaining ring) is used;
- With time, a keyway connection may appear very difficult to disassemble;
- Due to the “notch effect”, keyways create a stress point which compromises the shaft strength;
- Causes shaft unbalance;
- Load capacity and tolerance stack-up analyses are difficult to calculate and combine, therefore keyway connections are oversized;
- A lock is necessary to transfer axial forces.
Parallel key as per DIN6885
These are all products conforming to DIN 6885 standard requirements. This standard is published by the German Institute for Standardisation and its current version is DIN 6885-1:2021. Manufacturers of keys distributed by TME most commonly refer to this standard to ensure that all applicable requirements are met. Here, the available specification facilitates selecting a key according to the fit requirement, as this DIN standard also assumes 2 classes of fit: “tight” fit (P9) and “loose” fit (JS9 or N9).
Fitted keys and key fitting
A key is a prismatic element tightly fit within its lateral surfaces. As it does not exert pressure on wheel hub and shaft radial surfaces, it cannot be used to align these two components. As a rule, keyway connections are stationary connections, but they may also be used as moving connections (wheel hub movement along the shaft neck). For stationary connections, on lateral surfaces, the h6/P9 class fit is applied for a wheel hub and shaft. For moving connections, on lateral surfaces, the h6/D10 class fit is applied for a wheel hub and h6/H9 for a shaft.
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