- Huyett Marketing Department
- 04/26/2021
- Updated: 06/15/2023
Machine keys – also called shaft keys or key seats - are removable machine elements installed between shaft key seats and hub keyways to transmit torque in power transmission assemblies. They are also designed to protect a power transmission application by failing first, which protects other component elements.
Machine keys can be machined; finished in a variety of shapes, sizes, and materials; and tumbled by the thousands for automated assembly lines or just about any application. Though simple in design, they are a necessity in the transfer of power.
How Are Machine Keys Made?
Machine keys are cut from
Key Stock Material Selection Guide, which are square (or otherwise shaped) metal bars that are over one foot in length. Key stock can be used on its own in power transmission applications or in the field to replace broken shaft keys on site, but they are typically cut and machined into shaft keys.
For more information on key stock and their materials, read our
Key Stock Material Selection Guide.
Machine Key Design Elements
While they may appear to be simple fasteners, there are several important elements at play when designing shaft keys. The following section explains what these design elements are and how they ultimately affect application performance.
Shaft Key Profiles & Keyways
The profile of shaft keys affect installation, bearing stresses, and wear. Parts with square ends tend to be more difficult to install, but they provide more surface contact that round-end keys. Round end keys yield a greater coefficient of friction with the keyway, which reduces bearing stresses, but do not offer as much surface contact.
Because keyways are milled into the side of shafts using a rotating cutter, the entry and exit points of the cutter into the shaft form a rounded slot. This makes certain shaft keys – like Form A keys – match the pattern of the keyway which eases installation.
Types of Keyways
Machine keys can fit into a variety of keyways; the shaft key design will dictate the kind of keyway the application needs to have.
Profile or Closed
Milled along length of the shaft, but does not extend to end of the shaft
Sled Runner
Look like the runners of a sled with a round taper on one end
Woodruff or Hi-Pro
Cut along the length of the shaft with an arbor cutting tool
Typical examples of a machine key in a key shaft include:
Top fitting gib head keys ease assembly and disassembly.
Parallel keys located in key ways at the end of a shaft may protrude for full contact between key and shaft.
Key ways located along the length of a shaft allow for full contact in assembly with no way for the key to work itself out.
Woodruff keys are frequently used with tapered shafts. Although more difficult to install, they can not fall out of an assembly.
Types of Corners
Keyways may have filleted (rounded) corners to decrease stress concentration. However, shaft keys with square corners will not fully seat in filleted keyways.
Tolerance
Tolerance refers to the ways in which a shaft key can deviate from its nominal size. From the nominal size, it could deviate by being larger or smaller.
Oversize
The
actual
size will be at least the specified size and may be slightly larger than the
specified
size within the acceptable tolerance range.
Undersize
The
actual
size will be at least the
specified
size and may be slightly smaller than the specified size within the acceptable tolerance range.
Types of Fit
Fit literally refers to the way a shaft key fits into the keyway. Considering the different size options and the many different sizes of keyways, fit needs to be broken down into three distinct categories.
- An interference fit is produced when the machine key is slightly larger than the keyway.
- A clearance fit is produced when a space exists between the shaft key and the keyway.
- A transition fit is produced when either the shaft key clears the keyway or it collides with it.
Shear Analysis
Shear stress is a very important element in a shaft key’s design. A key’s shear strength refers to the key’s ability to withstand breaking or slicing along its planes; since a shaft key is designed to break before its mating power transmission components, it is vital to understand the shear stress it can endure and the point at which it will break.
To learn more about how a metal’s properties can influence its performance, read our
Introduction to Mechanical Properties of Metals.
Keys may fail if they are crushed due to excessive compression, bearing forces, or shear off in catastrophic failure.
Keys can also wear out or get distorted through fretting from high pressure and constant load.
Fretting
A machine key may wear out or get distorted from high pressure and/or constant load bearing.
Catastrophic Failure
If an application fails a machine key may shear or be crushed on load and will need to be replaced.
Machine Key Design Considerations
Considering the design elements of a shaft key, there are several points to consider before choosing the correct key for your application.
Shape / Type
Shaft keys are available in many different styles, and each one offers different benefits to accommodate a range of applications. Consider the impact of shape within your application’s keyway.
Function
Machine keys are designed to transfer torque between a gear and rotation shaft; it may shear under stress to protect people and expensive equipment. It may also need to be removed at some point; think about how your key needs to function in your application.
Location
Depending on the application, keyways are located at the end or along the length of the shaft. Location can impact key design and ultimately its performance.
Corner Designs
Sharp, square corners provide the most surface contact while radiused or chamfered corners are a little easier to install and fully seat in a keyway. Think about the desired ease of installation vs. the benefits of surface contact.
Material and Finish
Different materials and finishes will affect the shaft key’s overall performance by offering varying levels of strength, machinability, corrosion resistance, etc. Think about the environmental elements your shaft key will endure and how its metal properties will either contribute to or detract from your application.
For more information on how materials affect a metal’s overall performance, read our
Introduction to Types of Metal Finishes.
Types of Machine Keys
There are a wide variety of machine key types, all of which offer their own unique benefits. Some designs are based on how the parts are used, while others look at how the parts are manufactured or enhance efficiency in assembly. For best results, consult with a design engineer when developing a new application.
Here are some of the most common types of shaft keys, their key attributes and characteristics, and what they provide for your application.
Parallel Shaft Keys
Square and rectangle profiles are the most common forms for parallel keys. They are often referred to as straight keys.
Parallel shaft (or straight) keys are more common in the U.S. and cost less to produce than feathered keys. They are "side fitting," which means they fit edge to edge in the keyway.
Form B machine keys feature straight, 90° parallel sides and provide the most surface contact with both the key seat and the hub of a sprocket, gear, pulley, or cog.
Common Names:
Machine Keys, Sunk Keys, Key Stock
Applicable Standards:
ANSI B17.1, DIN 6885
Fabrication:
Cold drawn steel, cut to length, tumbled to remove burrs.
How to Identify:
Imperial is measured height x width x length. Metric is measured width x height x length.
Common Uses:
Original equipment, including motors, gear boxes, gear reducers, and transmissions.
Comments:
Square ends are more common in U.S.
Feathered Shaft Keys
Feathered shaft keys are parallel keys with round ends. They are more common in Europe. The round end design allows for the key to fully seat in the key way.
Form A machine keys have rounded end profiles, so the key fully fills a milled keyway. Some users suggest that the round design eases installation into the keyway.
Common Names:
Feather Key; Pratt & Whitney Key
Applicable Standards:
ANSI B17.1, DIN 6885
Fabrication:
Cold drawn steel, cut to length, both ends radiused.
How to Identify:
Imperial is measured height x width x length. Metric is measured width x height x length.
Common Uses:
Original equipment, including motors, gear boxes, gear reducers, and transmissions.
Comments:
Radiused ends are more common in Europe.
Huyett also offers
Form AB Machine Keys, which combine the two designs.
Gib Head Keys
Gib head keys are tapered machine keys that are hammered in place to secure pulleys and gears tightly on the shaft. The added feature of a head provides for easy removal. They are are designed for applications that require periodic maintenance or removal during disassembly.
There are a few key characteristics of gib head keys that are important to understand:
Typically the height is less than width. Gib head keys fit in the key way with "top and side fitting" which bears load on all sides.
In order to remove a gib head key with an extraction tool, a gap must be left between the head and the assembly.
Both the gib key and the hub key way are tapered.
Common Names:
Gib Head Key, Cotton Picker Key
Applicable Standards:
DIN 6884/6887, Industry norm
Fabrication:
Cold drawn steel, cut to length, machined tapered surface, tumble to deburr.
How to Identify:
Imperial is measured width x length. Metric is measured width x effective height x length. Commercial keys do not usually meet the ANSI specification.
Common Uses:
Original equipment, including motors, gear boxes, gear reducers, and transmissions. Farm equipment, especially cotton pickers, harvesters, and combines.
Comments:
Used to progressively install the key and take up slack from one access end. The head serves as a concussion point for hammering without damage to the shaft of the key. Common taper is 1/8" taper per foot.
Woodruff Keys
Woodruff keys, sometimes referred to as "half‑moon" keys because of their shape, are designed to install and pivot in the key way and will not fall out once assembled.
Woodruff keys are used to avoid milling a keyway near stress concentration prone shaft shoulders at the end of a shaft. With a woodruff key, the keyway does not extend to the end of the shaft so a stronger design is realized.
Proper Installation
Woodruff keys should be installed with the low side toward the hub. During installation, the key will rock in the keyway allowing for a tight fit.
If the key is installed with the high side to the hub, the key will catch causing improper installation and risk damage to the assembly.
Common Names:
Half‑moon Key, Round Key
Applicable Standards:
ANSI B17.1, DIN 6888
Fabrication:
Cold drawn profile, cut to width, grind, tumble.
How to Identify:
Imperial is measured diameter x width. Metric is measured width x height. Woodruff keys may have round or flat bottoms depending on specifications.
Common Uses:
Original equipment, where an arbor cutter is used to cut a key way that is half‑moon shaped.
Comments:
Alloy hardened keys usually have hash marks (small lines) embedded into the surface of the key along the top.
Hi-Pro Keys
Hi pro keys perform like woodruff keys but have the added benefit of "feet" that aid in installation by eliminating the potential for the key to rock in the keyway. This helps eliminate any play or movement during installation.
Common Names:
High Profile Keys
Applicable Standards:
ANSI B17.1, Industry norm
Fabrication:
Cold drawn profile, cut to width, grind, tumble.
How to Identify:
Width x length x lip height. Hi‑pro keys have small "feet" to keep them from rocking in the key way.
Common Uses:
Original equipment, where an arbor cutter is used to cut a key way that is half‑moon shaped.
Comments:
Alloy hardened keys usually have hash marks (small lines) embedded into the surface of the key along the top.
Special Order Machine Keys
Special Order Metric DIN 6885
Machine keys can be designed with special dimensional features like set screws, tapers, and other attributes. Mechanical features like tensile strength, shear strength, and wear resistance can be enhanced by material and secondary processes (like heat treating) that give parts added benefits.
Hex Shaft Keys and Round Machine Keys
Both hex and round shaft keys are available for special order.
Custom-Made Machine Keys
While machine keys are available in a wide range of standard sizes and profiles, many applications require custom machine keys. A custom machine key provides greater control over the fit, function, and fail point of the key for an application without the need to purchase or maintain expensive equipment to customize the machine keys in house.
If an application requires machine keys of a non standard size, type, tolerance, or material, Huyett can provide engineering support to help produce a print and manufacture and deliver them with short lead times.
Milled End
Radial Ends with Step
Key with Installed Pin
Chamfered Edges
E-Clip Groove with Turned End
Radial Ends with Holes
Huyett's Manufacturing Capabilities
Our extensive manufacturing capabilities allow us to manufacture custom machine keys with a wide variety of specifications. Capabilities include
cutting, turning, milling, drilling, threading, surface grinding, finishing, laser cutting, forming, wire forming,
and
stamping
. We also provide secondary processes like
heat treating, plating, coating,
and
marking
.
For more information on these capabilities and how Huyett can help you with custom machine keys, visit our
Custom Fastener Manufacturer Guide or our
Supply Chain Simplified: Complete In-House Manufacturing page.
How to Order Custom Machine Keys
Huyett custom manufactures machine keys of these types and others in your choice of size, material, and tolerance. Available materials include carbon steel, alloy steels, stainless steel, and specialty metals as required. We support order sizes ranging from one machine key to 1,000,000 keys per year, depending on the key style.
The custom key ordering process is as follows:
- Contact us with information about how many machine keys are needed along with a print or detailed description (or let our Engineering Support Team help develop a print for a custom application).
- A quote is typically available within eight business hours of the initial request.
- After the quote is approved and an order is sent, we provide a detailed production print for customer approval prior to final acceptance of the order.
- Once the order is accepted, we get to work manufacturing your machine keys.
In addition to manufacturing custom machine keys, we can also manage your key inventory and deliver them to your location via our just in time delivery service. To learn more about custom machine key manufacturing,
contact a member of our sales team today.
