Shafts, Keys, Keyseats, Circlips and Seals: ANSI B17.1 Depth Control Values (Selected)

ANSI B17.1 Depth Control Values (Selected)

British Standard Metric Keys and Keyways (BS 4235)

The British Standard provides metric dimensions and tolerances for keys and keyways, with three classes of fit for parallel keys and a single fit class for taper keys.

Three Fit Classes for Parallel Keys

• Free Fit: Applied when the hub must slide over the key during assembly. The keyway in the shaft uses H9 tolerance, and the hub uses D10 tolerance.
• Normal Fit: For mass-production assembly where the key is inserted with minimum fitting. Shaft keyway uses N9 tolerance, hub uses JS9 tolerance.
• Close Fit: For accurate fitting under maximum material conditions, which may involve component selection. Both shaft and hub keyways use P9 tolerance.

Key Material Specification

Keys complying with BS 4235 are manufactured from steel to BS 970 with a minimum tensile strength of 550 MN/m² in the finished condition. Qualifying steels include:

BS 4235 Keyway Dimensions — Square Parallel Keys (Metric)

BS 4235 Keyway Dimensions — Rectangular Parallel Keys (Metric)

BS 4235 Taper Key Dimensions (Metric)

Taper keys in the British metric standard are usually top fitting with a taper of 1:100 on the upper surface.

Square Taper Keys:

Rectangular Taper Keys (Selected):

BS 4235 Metric Key Tolerances

Square Parallel Keys:

Rectangular Parallel Keys:

Key Forms (BS 4235)

The standard defines three forms of parallel key:

• Form A: Both ends rounded
• Form B: One end rounded, one end square
• Form C: Both ends square

Preferred Lengths of Metric Keys (BS 4235 / ISO)

British Standard Imperial Keys and Keyways (BS 46)

While considered obsolescent, BS 46:Part 1:1958 (1985) remains widely referenced in legacy equipment and existing installations.

Key Applications by Type (BS 46)

Parallel Keys (rectangular and square):

• Used for transmitting unidirectional torques in transmissions not subject to heavy starting loads
• Used where periodic withdrawal or sliding of the hub member may be required
• Side fitting with top clearance
• Usually retained in the shaft more securely than in the hub
• Rectangular keys are the general-purpose type for shafts greater than 1 inch diameter
• Square keys are for shafts up to 1 inch diameter, or up to 6 inches where greater key depth is desired

Taper Keys (rectangular and square):

• Used for transmitting heavy unidirectional, reversing, or vibrating torques
• Used where periodic withdrawal of the key may be necessary
• Usually top fitting, but may be top and side fitting where required
• When top and side fitting, the hub keyway must have the same width as the shaft keyway
• Taper is 1:100 (same as metric standard)
• Rectangular taper for general purposes (less depth than square)
• Square taper for shafts up to 1 inch, or up to 6 inches where greater key depth is desired

Woodruff Keys (BS 46):

• For light applications or angular location of parts on tapered shaft ends
• Not recommended for heavy torque applications
• Corner radii in shaft and hub keyways are advisable to reduce stress concentration

Dimensions and Tolerances

The fitting allowance is designed to permit an interference between the key and the shaft keyway and a slightly easier condition between the key and the hub keyway. Any variation in the width of the keyway should be such that the greatest width is at the end from which the key enters. Any variation in the depth of the keyway should be such that the greatest depth is at the end from which the key enters.

Keys and keybar normally are not chamfered or radiused as supplied, but this may be done at the time of fitting. Corner radii are recommended for keyways to alleviate stress concentration.

BS 46 Rectangular Parallel Keys (Selected Sizes)

BS 46 Square Parallel Keys (Selected Sizes)

Stepped Shafts and Unequal Key Disposition

In stepped shafts, the larger diameters are usually required by considerations other than torque—for instance, resistance to bending. Where components such as fans, gears, impellers, etc., are attached to the larger shaft diameter, the use of a key smaller than standard for that diameter may be permissible.

However, this creates unequal disposition of the key in the shaft and its related hub. When using an undersized key, the dimensions H (shaft keyseat depth) and h (hub keyseat depth) must be recalculated to maintain the T/2 relationship (where T is the key thickness, split equally between shaft and hub).

Improvement method and result

Six months after the conveyor failure, the practitioner was on-site at a new grain processing facility. Same application. Same shaft size. Same operating conditions.

But this time, everything was different.

He started with the shaft diameter at the key location: 2 inches.

He looked up the standard key size: 1/2 × 3/8 inch rectangular key (ANSI B17.1, Table 1).

He specified Class 2 fit because the application involved reversing loads. This meant:

• Key stock, not bar stock (plus tolerance: +0.001 / −0.000)
• Side fit range: 0.001 inch interference to 0.002 inch clearance
• Top/bottom fit range: 0.000 to 0.035 inch clearance

He verified the keyseat alignment tolerances: offset within 0.010 inch, lead within 0.002 inch for the 3-inch keyseat length.

He specified a set screw: 1/2-inch diameter, with a flat machined on the shaft under the set screw location.

He checked the depth control values: S = 1.781 for the shaft, T = 2.161 for the hub (rectangular parallel configuration).

The conveyor ran for four years without a single key-related issue. Not because the practitioner used exotic materials or over-engineered the connection—but because he followed the standard exactly.

Key Selection Decision Framework

Use this decision tree every time you specify a key:

Step 1 — Determine Shaft Diameter at Key Location

• Measure or reference the drawing at the exact point where the key will sit
• For stepped shafts, use the local diameter, not the maximum

Step 2 — Select Key Type

• Shaft ≤ 6.5 in. → Square parallel key (preferred)
• Shaft > 6.5 in. → Rectangular parallel key (preferred)
• Reversing or vibrating loads → Taper key
• Light duty on tapered shaft end → Woodruff key
• Rod connections in tension/compression → Cotter

Step 3 — Look Up Key Size from Standard Table

• ANSI B17.1 for inch sizes
• BS 4235 for metric sizes
• Match shaft diameter range to key width and height

Step 4 — Select Fit Class

• Sliding hub required → Class 1 (clearance) or Free Fit
• General assembly → Class 2 (transition) or Normal Fit
• Critical alignment → Class 3 (interference) or Close Fit
• Taper keys → Class 2 (self-locking)

Step 5 — Specify Key Stock vs. Bar Stock

• Class 1 → Bar stock acceptable
• Class 2 → Key stock required
• Class 3 → Key stock with selective fitting

Step 6 — Verify Keyseat Dimensions

• Check depth control values (S and T)
• Verify alignment tolerances (offset and lead)
• Specify fillet radii if stress concentration is a concern
• Select set screw size if axial security is needed

Step 7 — Document Everything

• Key type, size, material, fit class
• Keyseat dimensions, tolerances, and alignment requirements
• Set screw specification (if applicable)
• Assembly and inspection procedures

Common Mistakes That Destroy Keys and Keyseats

Mistake 1: Using bar stock where key stock is required. Bar stock has negative tolerances (undersized). In a Class 2 fit application, it creates excessive clearance, leading to key rocking, wallowing, and eventual failure.

Mistake 2: Ignoring keyseat alignment tolerances. A keyseat offset of more than 0.010 inch or excessive lead creates uneven contact. The key bears on only one side, doubling the local stress and halving the fatigue life.

Mistake 3: Using a Woodruff key for heavy torque. The circular pocket removes significant shaft material. Under heavy or reversing loads, the shaft can crack at the keyseat root.

Mistake 4: Omitting set screws on parallel keys. Without axial restraint, parallel keys can "walk" out of their keyseats under vibration. Taper keys are self-retaining; parallel keys are not.

Mistake 5: Specifying a key by shaft OD on a stepped shaft. The key must be sized for the diameter at the key location, not the overall shaft diameter. An oversized key weakens the shaft more than necessary; an undersized key fails under load.

Mistake 6: Neglecting fillet radii in fatigue-critical applications. Sharp corners in keyseats are stress concentrators. In cyclic loading, cracks nucleate at these corners. Fillet radii dramatically improve fatigue life.

Mistake 7: Using shrink/force fit tolerances from the standard for hub keyways. In shrink and heavy force fits, it may be necessary to depart from the standard width and depth tolerances. The interference from the shrink fit can distort the keyway, requiring dimensional compensation.

Your Next Step

You now hold the complete reference for key and keyseat engineering—ANSI, British, metric, imperial, parallel, taper, Woodruff, and cotter. Every dimension. Every tolerance. Every fit class.

Here is your challenge: Pull out the last assembly drawing you worked on that includes a keyed shaft connection. Check every specification against the tables in this guide. Verify the key size matches the shaft diameter at the key location. Confirm the fit class matches the application loading. Inspect the keyseat alignment tolerance call-out.

If everything checks out, you've been doing it right.

If even one specification is off, you just found the failure waiting to happen—before it shut down a production line.

That's worth more than any conveyor.

This guide references ANSI B17.1-1967 (R1998), ANSI B17.2-1967 (R1998), BS 4235:Part 1:1972 (1986), and BS 46:Part 1:1958 (1985). All dimensional data should be verified against current editions of these standards for critical applications.

Failure trigger and engineering context

Before the practitioner could understand why her shaft failed, the practitioner made her recite the definition until it was burned into her memory:

A shaft is a rotating member supported by bearings that transmits torque (and power).

Simple. Deceptively simple. But within that definition hide layers of complexity that separate competent engineers from the ones who get called at 3 AM.

What You Need to Know About Shafts

• The rotation may be continuous or intermittent, uni-directional or reversing. A motor shaft spins one way; a vehicle axle reverses every time you back up.
• Shafts to which wheels are attached are called axles. An axle carries a wheel. A shaft carries power. Sometimes the same piece of metal does both.
• Shafts are usually circular — solid or hollow. Square section shafts exist for special applications but are uncommon in general power transmission.
• Shafts are usually rigid. Flexible shafts (cables) exist in tools like engraving devices and dentist's drills, but this guide deals exclusively with rigid shafts.
• The shaft is usually relatively long compared to its diameter. This is the geometric reality that makes bending such a dominant concern.
• In engineering, shafts are usually made of steel (or other metal), though non-metallic shafts appear in special applications.
• Torque and power are transmitted from one location on the shaft to another. Input typically comes from a motor or engine. Output goes to gears, pulleys, sprockets, or directly to the load.

The critical insight the practitioner missed the first time: A shaft isn't just carrying torque from point A to point B. It's simultaneously resisting bending from the weight of every component hanging off it, absorbing shock loads from process variations, and surviving millions of stress reversals as it rotates.

That's three jobs at once. And failing at any one of them means failure at all of them.

The Three Loads That Attack Every Shaft

the practitioner pulled a whiteboard marker and drew three arrows on the break room board.

"Every force on a shaft," he said, "falls into one of three categories."

Steady Loads

These are the primary design loads — forces that occur relatively continuously during operation:

The dangerous truth: These loads can — and often do — occur simultaneously. Your shaft might be twisting from motor torque, bending from sprocket weight, and carrying axial thrust from a helical gear all at the same time.

Shock Loads

Shock loads are intermittent. They spike suddenly and disappear.

Picture a rolling mill: the moment the metal billet first contacts the roll, there's a sudden, massive increase in load on the roll shaft. A punching press creates the same effect when the punch first contacts the metal.

Shock loads are common in engineering. Even though they may occur only momentarily, your shaft must be designed to withstand them. Shear pins and overload protection devices exist precisely because shock loads can be catastrophic, but the shaft itself still needs to survive them.

Key fact for electric motors: Motors are usually fitted with overload protection so the motor cuts out before shaft damage occurs. But the shaft's overload capacity depends on the steel — and the design.

Inertia Loads

Every time a shaft speeds up or slows down, inertia loads appear. Their magnitude depends on two factors:

• The rapidity of the acceleration or deceleration (how fast you're changing speed)
• The mass moment of inertia of the shaft itself and everything attached to it — gears, pulleys, flywheels, couplings, and any connected equipment

For a shaft running at relatively steady speed, inertia loads only matter during start-up and shutdown. But whether they're a minor nuisance or a major design concern depends entirely on how the motor starts:

the practitioner's shaft was on a hard-start motor. Every single morning when the plant powered up, that shaft experienced torque spikes of up to five times normal. For eleven months. Thousands of start cycles. And nobody had checked.

The Two Ways Shafts Die

the practitioner held up the broken shaft and pointed to two different zones on the fracture surface.

"There are exactly two ways a shaft fails," he said. "One is dramatic. The other is a silent assassin."

Failure Mode 1: Excessive Load (The Dramatic One)

This happens when stress exceeds the yield strength — usually from an extreme overload event. Foreign objects jamming in machinery. Catastrophic bearing seizure. Something that was never supposed to happen, happening.

It's relatively rare in well-engineered systems because load-limiting devices (shear pins, overload clutches, motor protection) typically prevent it.

If overload occurs and the shaft is not broken or cracked, it may still be serviceable. But — and this is critical — for design purposes, the shaft should be designed to prevent stress above the yield stress. If permanent deformation has occurred, the shaft has failed, even if it's still spinning.