A single key in a keyway is fine for modest torque, but when you need to transmit serious power through a shaft-to-hub connection — think driveshafts, transmission gears, or CV joints — you switch to a spline. Splines are essentially many parallel keys machined integrally into the shaft, distributing torque across dozens of engagement surfaces simultaneously. This lesson focuses on the design tradeoffs that separate spline types, since we've already covered keyways.

The three dominant profiles:

• Straight-sided (parallel) splines: Rectangular teeth, simple to broach. Common in older machinery and agricultural PTOs. Poor stress distribution at tooth roots — stress concentrations limit fatigue life.
• Involute splines: Same tooth profile as gear teeth (typically 30°, 37.5°, or 45° pressure angle). The involute shape allows teeth to self-center under load and distributes stress more gracefully. Dominant in automotive and aerospace. Standardized under ANSI B92.1 and ISO 4156.
• Serrated (small triangular teeth): Many small teeth for permanent, non-sliding connections — steering wheel to column, for example. Not for power transmission at rated speed.

Fit types matter as much as profile:

• Side fit: Teeth flanks bear the load; clearance at major/minor diameter. Most common for torque transmission.
• Major diameter fit: Centered by the tooth tips. Used when concentricity matters more than torque capacity.
• Minor diameter fit: Centered at the root. Rare, used for specific alignment requirements.

Real-world example: A rear-wheel-drive car's driveshaft uses a sliding involute spline at one end to accommodate suspension travel — as the axle moves up and down, the shaft length changes by 25–50 mm, and the spline slides while still transmitting full engine torque. Without splines, you'd need a telescoping design with far worse torque capacity and durability.

Rule of thumb for torque capacity (straight-sided splines, assuming 25% tooth engagement to account for manufacturing tolerances):

T ≈ 0.25 × N × h × L × r_m × σ_allow

where N = number of teeth, h = tooth height, L = engaged length, r_m = mean radius, σ_allow = allowable bearing stress (typically 100 MPa for hardened steel). A 10-tooth spline with 3 mm tooth height, 25 mm length, 15 mm mean radius yields roughly 280 N·m capacity.

Design gotcha: Sliding splines under load suffer from fretting wear — micro-motion at the tooth contact strips lubricant and accelerates fatigue. Molybdenum disulfide grease or PTFE coatings are standard countermeasures on axial-sliding splines.