Shafts, Keys, Keyseats, Circlips and Seals: The Conveyor That Stopped a Factory

The Conveyor That Stopped a Factory

the practitioner had built conveyor systems for eleven years. He could size a drive motor in his sleep, select bearings with his eyes closed, and calculate chain tensions while eating lunch. But on a Tuesday morning in a grain processing facility, a 2-inch diameter shaft inside a gearbox coupling began to vibrate. By Wednesday, the key had wallowed out the keyseat. By Thursday, the shaft was spinning freely inside the hub, the entire conveyor line was dead, and 14,000 units of product sat motionless on the warehouse floor.

The root cause wasn't the motor. It wasn't the gearbox. It wasn't even the shaft.

It was a key that was 1/16 of an inch too narrow for the keyseat, installed with a Class 1 clearance fit where a Class 2 fit was required.

the practitioner had used bar stock instead of key stock. The fit was loose. Under reversing loads, the key rocked back and forth thousands of times per minute, gradually hammering the keyseat walls wider until positive torque transmission was gone.

Total cost of the failure: three days of lost production, emergency machining of a new shaft, expedited shipping of a replacement coupling, and a client who now questioned every specification the practitioner had ever written.

This is the story of how a small rectangular piece of steel—one of the simplest machine elements in existence—can make or break an entire mechanical system. And more importantly, this is the guide that ensures you never make the same mistake.

What Exactly Are Keys and Keyseats?

Before you can master them, you need absolute clarity on what these components are and what they do.

Key: A demountable machinery part which, when assembled into keyseats, provides a positive means for transmitting torque between the shaft and hub.

Keyseat: An axially located rectangular groove in a shaft or hub.

That's it. Two components. One function: transmit rotational force (torque) from a shaft to a hub—or from a hub to a shaft—reliably, repeatedly, and without failure.

Every gear, every pulley, every coupling, every flywheel that sits on a rotating shaft and needs to turn with it depends on this connection. Get it right, and the assembly runs for decades. Get it wrong, and you're the practitioner on a Thursday morning.

The Governing Standards You Must Know

The engineering world doesn't leave key and keyseat design to guesswork. Two primary standards control dimensions, tolerances, and fit classes:

• ANSI B17.1-1967 (R1998) — Keys and Keyseats (inch-based, parallel and taper keys)
• ANSI B17.2-1967 (R1998) — Woodruff Keys and Keyseats (inch-based, semicircular keys)
• BS 4235:Part 1:1972 (1986) — British Standard Metric Keys and Keyways (metric-based, parallel and taper)
• BS 46:Part 1:1958 (1985) — British Standard Imperial Keys and Keyways (inch-based, obsolescent but still referenced)
• ISO R773:1969 — Rectangular or Square Parallel Keys (metric)
• ISO R774:1969 — Taper Keys with or without Gib Head (metric)

The sizes and tolerances in these standards are for single key applications only. Dual-key configurations require separate engineering analysis.

The Key Family: Every Type You Need to Understand

Not all keys are created equal. Each type exists because a specific engineering problem demanded a specific solution.

Square Parallel Keys

The workhorse. Square cross-section, uniform width and height, parallel sides (no taper). These are the default choice for shaft diameters up through 6½ inches. They transmit torque through side contact between the key and both the shaft keyseat and the hub keyway.

When to use them: General-purpose torque transmission where the hub does not need to slide axially on the shaft under load.

Rectangular Parallel Keys

The heavy-duty alternative. Wider than they are tall, providing greater shear area. Preferred for shaft diameters above 6½ inches, and for any application where a greater key width is needed relative to the key height.

When to use them: Large shafts, high-torque applications, and situations where a square key's height would be excessive relative to the shaft wall thickness.

Taper Keys

The self-locking solution. These keys have a 1:100 taper on the top surface (approximately ⅛ inch per foot). When driven into place, the taper creates a wedging action that locks the hub axially on the shaft. They are top fitting—torque is transmitted through the side faces, while the taper creates axial clamping force.

When to use them: Heavy unidirectional, reversing, or vibrating torques, and applications where periodic withdrawal of the key may be necessary.

Gib Head Keys

Taper keys with an extraction feature. The gib head is a projecting head at the large end of the taper key that provides a surface against which a wedge or extractor tool can be driven to remove the key. Critical for applications where the key cannot be driven out from behind.

Woodruff Keys

The self-aligning specialist. Semicircular in shape, these keys sit in a circular pocket (keyseat) milled into the shaft. The curved bottom allows the key to rock slightly and self-align with the hub keyway, compensating for minor angular misalignment.

When to use them: Light-duty applications, angular location of parts on tapered shaft ends, and situations where self-alignment is more important than heavy torque capacity.

When NOT to use them: Heavy torque transmission. The circular pocket significantly weakens the shaft.

Cotters

The tension/compression connector. A cotter is a form of key used to connect rods subjected to tension, compression, or both. The cotter experiences shearing stresses at two transverse cross-sections.

• Plain cotters (friction-retained): Taper of ¼ to ½ inch per foot
• Secured cotters (set-screw retained): Taper of 1½ to 2 inches per foot

Key Size Versus Shaft Diameter: The Selection Table That Governs Everything

This is the single most important reference table in key design. It establishes the mandatory relationship between your shaft diameter and the key size you must use.

Critical rule: For a stepped shaft, the size of a key is determined by the diameter of the shaft at the point of location of the key—not the maximum shaft diameter.

ANSI B17.1 Key Size Selection (Inch)

The dividing line at 6½ inches is not arbitrary. Above that diameter, rectangular keys provide the optimal balance of shear strength and shaft wall retention. Below it, the equal depth of square keys gives the best torque transmission per unit of key width.

What If You Need a Shallower Hub Keyseat?

The standard explicitly addresses this: If special considerations dictate the use of a keyseat in the hub shallower than the preferred nominal depth, it is recommended that the tabulated preferred nominal standard keyseat always be used in the shaft.

In other words, you can compromise the hub—but never the shaft.

Understanding Fit Classes: Why the practitioner' Conveyor Failed

This is where the real engineering lives. Key dimensions alone don't determine success. The fit between key, shaft keyseat, and hub keyway determines whether your assembly transmits torque reliably or beats itself to death.

The Two Types of Key Stock

The ANSI standard recognizes two classes of key stock used in industry:

• Key stock: Close, plus-toleranced material (tighter, precision-ground)
• Bar stock: Broad, negative-toleranced material (looser, general-purpose)

This distinction directly determines which fit class you achieve. Using bar stock where key stock is required was exactly the practitioner' mistake.

ANSI Fit Classes Explained

Class 1 Fit — Clearance (Bar Stock)

• Side fit: Clearance or metal-to-metal
• Applies to parallel keys only
• Uses bar stock keys with corresponding keyseat tolerances
• This is a relatively free fit

Class 2 Fit — Transition (Key Stock)

• Side fit: Possible interference or clearance
• Applies to parallel and taper keys
• Uses key stock with corresponding keyseat tolerances
• This is a relatively tight fit

Class 3 Fit — Interference

• Side fit: Interference
• Not tabulated in the standard since the degree of interference has not been standardized
• Suggested approach: Use the top and bottom fit range from Class 2 for parallel keys

ANSI Standard Key Width Tolerances

Parallel Key Stock (Plus Tolerance):

Parallel Bar Stock (Negative Tolerance):

Taper Keys (Plain or Gib Head):

Detailed Fit Ranges: Class 1 and Class 2

Understanding the actual clearance and interference values is critical for predicting assembly behavior.

Class 1 Fit — Parallel Keys (Bar Stock)

Class 2 Fit — Parallel and Taper Keys (Key Stock)

CL = Clearance; INT = Interference. These are limits of variation.

Notice the critical difference in taper keys: The top and bottom fit range shows 0.025 interference on the tight end. This is the wedging action that locks taper keys in place. Parallel keys, by contrast, always maintain clearance on top and bottom.

Gib Head Keys: Dimensions for Extraction

When a taper key is buried inside an assembly where you cannot drive it out from behind, the gib head provides the extraction surface. These dimensions are standardized.

ANSI Standard Gib Head Nominal Dimensions

For sizes larger than tabulated: The standard suggests these relationships as a guide:

$$A = 1.8 \times H$$

$$B = 1.2 \times H$$

Keyseat Fillet Radii and Key Chamfers: Controlling Stress Concentration

In general practice, chamfered keys and filleted keyseats are not commonly used. However, fillets in keyseats decrease stress concentration at corners. When used, fillet radii should be as large as possible without causing excessive bearing stresses due to reduced contact area between the key and its mating parts.

Keys must be chamfered or rounded to clear fillet radii.

Suggested Fillet Radius and Key Chamfer (ANSI B17.1)

These values assume general conditions and should be used only as a guide when critical stresses are encountered.

Keyseat Tolerances for Electric Motors and Generators

Electric motor and generator shaft extensions have their own tolerances, tighter than general-purpose applications because motor shafts often need to accept couplings and pulleys from various manufacturers without custom fitting.

ANSI B17.1 Motor/Generator Keyseat Tolerances

Set Screws for Use Over Keys

Set screws are frequently used in conjunction with keys to provide additional security against axial movement of the hub on the shaft. The ANSI standard provides recommended set screw diameters based on shaft size and key width.

ANSI B17.1 Set Screw Selection Guide

These set screw diameter selections are offered as a guide, but their use should be dependent upon design considerations. A flat on the shaft under the set screw is advisable for better grip and to prevent shaft damage.

Keyseat Alignment Tolerances: The Hidden Specification

Many engineers focus exclusively on key dimensions and fit classes, but keyseat alignment tolerances are equally critical. Misaligned keyseats cause uneven load distribution, accelerated wear, and premature failure.

ANSI B17.1 Alignment Requirements

Offset tolerance (parallel displacement of keyseat centerline from centerline of shaft or bore):

• Maximum offset: 0.010 inch for all keyseat sizes

Lead tolerance (angular displacement of keyseat centerline, measured at right angles to shaft centerline):

Woodruff Keys: The Self-Aligning Solution

The Woodruff Key Numbering System

The key number tells you the exact dimensions. The last two digits give the nominal diameter (B) in eighths of an inch. The digits preceding the last two give the nominal width (W) in thirty-seconds of an inch.

Example: Key No. 808

• Last two digits: 08 → Diameter B = 8/8 = 1 inch
• Preceding digits: 8 → Width W = 8/32 = 1/4 inch
• So Key No. 808 = 1/4 × 1 inch Woodruff key

Example: Key No. 1210

• Last two digits: 10 → Diameter B = 10/8 = 1¼ inch
• Preceding digits: 12 → Width W = 12/32 = 3/8 inch
• So Key No. 1210 = 3/8 × 1¼ inch Woodruff key

Woodruff Key Definitions

• Woodruff Key: A demountable machinery part which, when assembled into keyseats, provides a positive means for transmitting torque between the shaft and hub.
• Woodruff Key Number: An identification number by which the size of key may be readily determined.
• Woodruff Keyseat—Shaft: The circular pocket in which the key is retained.
• Woodruff Keyseat—Hub: An axially located rectangular groove in a hub.
• Woodruff Keyseat Milling Cutter: An arbor-type or shank-type milling cutter normally used for milling Woodruff keyseats in shafts.

ANSI Standard Woodruff Keys — Selected Sizes (ANSI B17.2)

Actual Length (F) tolerance: +0.000 / −0.010 inch for all Woodruff keys.

Woodruff Keyseat Dimensions (Selected, ANSI B17.2)

Finding Depth of Keyseat and Checking Assembled Keys

The Milling Formula

For milling keyseats, the total depth to feed the cutter in from the outside of the shaft to the bottom of the keyseat is:

$$\text{Total Feed Depth} = M + D$$

where D is the depth of the keyseat, and M is the distance between the top of the shaft and a line passing through the upper corners of the keyseat.

M is calculated by:

$$M = \frac{1}{2}\left(S - \sqrt{S^2 - E^2}\right)$$

where S is the shaft diameter and E is the width of the keyseat.

Simplified approximation (accurate to within 0.001 inch):

$$M \approx \frac{E^2}{4S}$$

The Assembly Check Formula

For checking an assembled key and shaft, the caliper measurement J between the top of the key and the bottom of the shaft is:

$$J = S - (M + D) + C$$

where C is the height of the key above the keyseat.

For Woodruff keys, dimensions C and D can be found in the Woodruff key tables. Assuming shaft diameter S is normal size, the tolerance on dimension J for Woodruff keys in keyslots is +0.000 / −0.010 inch.

Depths for Milling Keyseats — Reference Table (Selected Values)

The following table gives dimension M for various shaft diameters and keyseat widths:

Depth Control Values S and T for Shaft and Hub

The ANSI standard provides depth control values for verifying proper keyseat depth during machining and inspection. These values represent the distance from the opposite side of the shaft (or bore) to the bottom of the keyseat.