Spline shafts are mechanical components that provide anti-rotation when functioning as a linear guide or otherwise transmit torque. The splines, grooves, or tongues are machined along the length of the shaft. They mate with a female-splined bore, gear, bearing, or other mating component. They transfer rotary motion in rotary drive applications or function as a linear guide when paired with an internally-splined bearing or bushing.

Specifications

Specifications for splined shafts include the type of teeth or grooves, materials of construction, diameter, length, and features.

Types

Spline shafts differ by the types of grooves or tongues that transmit torque.

• Parallel key splines have ridges with a square profile.
• Involute splines contain tapered ridges that decrease stress concentration.
• Crowned splines have modified teeth that taper towards the end face to allow for angular misalignment.
• Serrations have a tooth form that is non-involute. Teeth are in the form of an included angle that allows for more teeth on a smaller diameter shaft.

• Helical splines have either parallel or involute teeth. The ridges form a helix pattern about the shaft. They allow for rotary and linear motion and also minimize stress concentrations for a stationary joint under high load.

Materials

Splined shafts are generally manufactured from metallic elements due to localized stresses observed between the teeth or tongues of the shaft. The following materials are the most common.

Alloy steels are ferrous alloys based on iron, carbon, and alloying elements such as chromium, molybdenum, vanadium, and nickel. Alloy steels include hardenable high-alloy steels, high-strength low-alloy steels, maraging steels, and other specialty steel alloys.

Aluminum alloys provide high toughness at moderate strength levels, with good corrosion resistance and less-than-half the density of steel.

Carbon steel is a commercial iron that contains carbon in any amount up to about 1.7 percent as an essential alloying constituent. It is malleable under suitable conditions and distinguished from cast iron by its malleability and lower carbon content.    

Stainless steels are highly corrosion resistant, ferrous alloys that contain chromium and/or nickel additions.

Shaft diameter

The diameter is a nominal measurement across the shaft and is manufactured to close tolerances as it mates with a matching component. The shaft diameter is never reduced more than is necessary to permit easy slip-fitting of components.

Length

Most shafts are manufactured to standard lengths and can be cut to a specific length to match customer needs. Shafts should be ordered at a length that exceeds the length of its intended use.

Features

• Chamfered: To prevent stress concentrations the ends of the splines are cut at an angle or beveled.
• Shaft/hub assembly: Internal and external splines are packaged together and offered as an assembly.

Standards

ISO 14 -- Straight-side splines fir cylindrical shafts with internal centering

ANSI B92.1 -- Sizes, performance, and inspection of involute splines

DIN 5480-1 -- Reference diameters for involute splines and spline joints 

Resources

Wikipedia—Spline

Gear Solutions—Inside splines

Engineer's Edge—Spline engineering design formula 

Image Credits:

Grob Inc.