Active learning ideas
Keys and Cotters are temporary fasteners used to connect shafts to hubs or to join rods. This topic covers various types of keys (sunk, saddle, gib-head) and the cotter joint. Students learn about the 'taper', a slight slope that allows these parts to be driven in tightly. This is a critical lesson in mechanical advantage and friction-based fastening.
Activity 01
Using wooden dowels with pre-cut slots and small wooden wedges, students try to fix a 'hub' (a cardboard disc) to the shaft. They compare how a flat key versus a tapered key holds the disc in place under rotation.
What is the taper ratio used in a standard cotter?
Activity 02
Students analyze why a sunk key requires a slot in both the shaft and the hub, while a saddle key only needs one in the hub. They discuss which one can transmit more power and why.
How does a sunk key differ from a saddle key?
Activity 03
Students draw a cotter with a 1:30 taper. Peers use a 'slope check' (measuring the width at both ends) to see if the taper is drawn to the correct mathematical proportion.
How are keys represented in a shaft cross-section?
A few notes on teaching this unit
Watch Out for These Misconceptions
Drawing the taper on both sides of a cotter.
A standard cotter usually has a taper on only one side to simplify the machining of the slots. Peer-led sketching of the 'socket and spigot' assembly helps students see how the straight side of the cotter rests against one part while the tapered side wedges against the other.
Forgetting that a sunk key is half in the shaft and half in the hub.
Students often draw the key entirely inside the shaft. Using a cross-sectional physical model (like a cut-away pipe) helps them visualize the 'keyway' and the 'keyseat' and how the key bridges the two parts.
Methods used in this brief
Standard Profiles of Screw Threads
Drafting standard thread profiles such as BSW, Metric, and Square threads to scale.
8 methodologies
Nuts and Bolts
Orthographic projection of hexagonal and square nuts and bolts.
8 methodologies
Washers, Rivets, and Riveted Joints
Drafting plain washers, various rivet heads, and standard riveted joints.
8 methodologies