A machine is only as reliable as the joints between its parts. Keys lock gears and pulleys to their shafts, couplings join one shaft to another, and bolts, screws and pins hold everything else together. These are mostly standard, catalogue parts, but choosing and sizing them correctly is still the designer’s job.
Keys and splines
A key is a small steel bar that sits half in a slot (keyway) in the shaft and half in a slot in the hub, so the hub can’t turn on the shaft.
| Type | Features | Typical uses |
|---|---|---|
| Parallel key (ISO 773 / DIN 6885) | Rectangular, the most common; hub slides on axially | Gears, pulleys and couplings on general shafts |
| Woodruff key | Half-moon shape in a curved seat; self-aligning | Small and tapered shafts, light torque |
| Taper and gib-head keys | Driven in tight; also hold the hub axially | Older and heavy machinery |
| Splines | Many teeth around the shaft; the hub can slide | Gearbox shafts, drive shafts, high torque |
| Keyless (shrink fit, locking assembly) | Friction from clamping; no keyway to weaken the shaft | High torque, reversing loads, precise positioning |
The key size comes from the shaft diameter in the standard table (for example, an 8 × 7 mm key for shafts from 22 to 30 mm). The designer then chooses the key length and checks two stresses:
| Symbol | Meaning | Unit |
|---|---|---|
| Shear stress in the key | Pa | |
| Crushing (bearing) stress on the side of the key | Pa | |
| Torque | N·m | |
| Shaft diameter | m | |
| , | Key width and height | m |
| Key length in contact | m |
Worked example: checking a parallel key
A 30 mm shaft carries 96.8 N·m through an 8 × 7 mm parallel key, 40 mm long.
Crushing on the hub side usually limits the key first. Make the key longer, not wider, if it is overloaded; the width is fixed by the standard.
Couplings
A coupling joins two shafts, typically a motor to a gearbox or a gearbox to a pump, and transmits the torque between them. The main question is how much misalignment it must accept. Two shafts are never perfectly lined up: they can be offset (parallel misalignment), at an angle (angular misalignment) or move along their axis (axial float).
| Type | Misalignment | Features | Typical uses |
|---|---|---|---|
| Rigid (sleeve, flange) | None | Simple, stiff; shafts must be accurately aligned | Line shafts, precise drives on a common base |
| Jaw (spider) coupling | Small | Elastomer insert damps shocks; fail-safe | Motors to pumps and gearboxes |
| Disc and bellows | Small, zero backlash | Torsionally stiff | Servo motors, encoders, CNC axes |
| Gear coupling | Moderate | Very high torque in a small size | Heavy industrial drives |
| Oldham coupling | Larger parallel offset | Sliding centre disc | Light drives with offset shafts |
| Universal (Hooke’s) joint | Large angles | Speed fluctuates unless used in pairs | Vehicle drive shafts, steering columns |
Threaded fasteners
Bolts, screws and studs
A bolt passes through clearance holes and is tightened with a nut. A screw threads into a tapped hole in one of the parts. A stud is threaded at both ends, fixed in a tapped hole, and takes a nut, which is useful where a part is removed often. Metric threads are written as M10 (coarse pitch, 1.5 mm) or M10 × 1.25 (fine pitch).
Strength grades
The grade is stamped on the head. For metric steel bolts (ISO 898-1), the two numbers give the strength: in 8.8, the first 8 means a tensile strength of 800 MPa and “.8” means the yield is 80 % of that, 640 MPa.
| Grade | Tensile strength | Yield strength | Typical use |
|---|---|---|---|
| 4.6 | 400 MPa | 240 MPa | Light, non-critical joints |
| 8.8 | 800 MPa | 640 MPa | General machine building |
| 10.9 | 1000 MPa | 900 MPa | Highly loaded joints, vehicles |
| 12.9 | 1200 MPa | 1080 MPa | Socket head cap screws, tooling |
| A2-70 / A4-70 (stainless) | 700 MPa | 450 MPa | Corrosive environments |
Preload: why bolts are tightened hard
When a bolt is tightened, it stretches slightly and pulls the parts together with a clamping force called the preload. As long as the preload is greater than the forces trying to separate the joint, the parts stay in contact. Friction between them carries sideways loads, and the bolt sees only a small part of any changing load, which protects it from fatigue. Most bolted joints that fail were not tightened enough.
A common target is a preload of about 70 % of the bolt’s yield load, on the thread’s tensile stress area . The tightening torque follows from:
where is the nut factor, about 0.2 for plain, lightly oiled steel threads.
Worked example: tightening an M10 bolt
Find the preload and tightening torque for an M10 grade 8.8 bolt (A_t = 58.0 mm²).
Lubricated or plated threads need less torque for the same preload, so always use the torque value for the actual thread condition, from the fastener maker or a standard such as VDI 2230.
Keeping fasteners tight
Vibration can loosen bolts. Proper preload is the first defence; after that, use thread-locking adhesive, prevailing-torque (nylon-insert) nuts, wedge-locking washers or safety wire. Plain spring washers do little and are no longer recommended for critical joints.
Pins and rivets
- Dowel pins locate two parts precisely (reamed holes, light press fit), so they go back together in the same position. Bolts clamp; dowels locate.
- Spring (roll) pins are slotted tubes that grip in a drilled hole; used for levers, hinges and collars.
- Clevis pins with split pins or circlips form hinge joints.
- Rivets make permanent joints in sheet metal; blind (pop) rivets need access from one side only.
Key points
- Keys pass torque from shaft to hub; size from the standard table, then check shear and crushing over the key length.
- Choose a coupling by the misalignment it must take and how stiff the drive must be.
- Bolt grade 8.8 means 800 MPa tensile and 640 MPa yield.
- Preload is what makes a bolted joint work; tighten to a calculated torque for the actual thread condition.