Power Transmission Guide

ANSI No.	Pitch (in)	Max power @ 100 RPM (est.)
40	0.500	~2 kW
50	0.625	~4 kW
60	0.750	~6 kW
80	1.000	~11 kW
100	1.250	~18 kW
120	1.500	~27 kW

ISO equivalent: ISO 08B ≈ ANSI 40, ISO 10B ≈ ANSI 50, etc. (metric pitch, slightly different dimensions).

Minimum teeth (small sprocket): 17-19 teeth for smooth running. Below 17 → increased chordal action (speed variation), noise, and accelerated wear.

Maximum ratio (single stage): 7:1 typical, up to 10:1 possible with care.

Chain length: should be an even number of pitches to avoid using an offset link (weaker than a standard link).

Center distance: ideal is 30-50 pitches. Too short → rapid wear and rough running. Too long → chain whip/vibration.

Wear mechanism: pin-bushing wear increases effective pitch length → chain "grows."

Replace when: chain has elongated 1.5% (light duty) to 3% (before it starts riding up on sprocket teeth — "tip on the tooth" failure).

Measuring elongation: measure a length of chain (e.g., 12 pitches) under load and compare to nominal.

Lubrication is critical: proper oil (not grease, which doesn't penetrate pin-bushing interface) can extend chain life 5-10×.

✅ High torque at low speed (conveyors, agricultural equipment)
✅ Contaminated/oily environment (belt would slip)
✅ Exact speed ratio needed without slip
✅ Long service life expected with maintenance

❌ High speed applications (chain "whip" and noise increase)
❌ Where lubrication is impossible (food/clean environments — use belt instead)
❌ Shock-sensitive drivetrain (belt absorbs shock better)

Zero misalignment tolerance!

Types: sleeve, clamp (split), flange.

Use when: shafts are perfectly aligned and will stay that way (short, rigid frame machines).

Danger: any misalignment (angular, parallel, or axial) transmits directly to bearings → premature bearing failure, vibration, possible shaft fracture.

Alignment tolerance: essentially zero — use dial indicators or laser alignment tools before final tightening.

Jaw coupling (spider): elastomer spider between two hubs. Accommodates ~0.5° angular, small parallel misalignment. Absorbs shock and vibration. Element wears out over time (replaceable).

Oldham coupling: two hubs + floating center disc. Handles parallel misalignment well, ZERO backlash — good for servo/precision motion.

Disc coupling: thin metal discs flex to accommodate misalignment. High torsional stiffness (good for servo), handles all 3 misalignment types.

Gear coupling: internal/external gear teeth allow sliding. High torque capacity, needs lubrication.

Bellows coupling: metal bellows, zero backlash, low inertia — precision motion control.

Parallel (offset): shafts parallel but not concentric. Typical tolerance: 0.05-0.5mm depending on coupling type.

Angular: shaft centerlines intersect at an angle. Typical tolerance: 0.5-3° depending on type.

Axial (end float): shafts move toward/away from each other (thermal growth!). Most couplings accommodate a few mm.

1. Torque — with service factor (1.5-2× for shock loads)
2. Speed (RPM) — check max speed rating
3. Misalignment — expected parallel/angular/axial values
4. Bore sizes — both shaft diameters
5. Backlash requirement — zero for servo/precision, tolerant for general power transmission
6. Environment — temperature, chemicals (elastomer compatibility)
7. Shock/vibration — favor elastomeric types for damping

Sizing rule of thumb (metric, DIN 6885):

Shaft ⌀ (mm)	Key W×H (mm)
10-12	4×4
17-22	6×6
30-38	10×8
44-50	14×9
58-65	18×11

Key length: typically 1.0-1.5× shaft diameter, but check shear/bearing stress for actual loads.

Shear failure: key shears across its cross-section — check τ = F/(L×W) against allowable shear stress.

Bearing/crushing failure: key or keyway side wall deforms — check σ = F/(L×H/2) against allowable bearing stress.

By design: keys are often intentionally the "weak link" — sized to shear before more expensive components (gearbox, motor shaft) are damaged in an overload event. This is a deliberate safety design choice in some applications.

Fretting at keyway: a common fatigue initiation site — sharp corners in the keyway are stress concentrators (Kt up to 2-3).

Involute splines: like small internal/external gear teeth. Distribute load over many teeth (vs. one key) → much higher torque capacity in same diameter.

Advantages over keys:
• Self-centering (no eccentricity)
• Higher torque density
• Better fatigue resistance (no single stress riser)
• Can allow axial sliding under load (splined shafts)

Common applications: automotive drivetrains, PTO shafts, gearbox output shafts, aerospace actuators.

DIN 5480 designation example: W 25×1.25×18 — reference diameter 25mm, module 1.25, 18 teeth.

For permanent or semi-permanent connections without a key at all:

Press fit: mechanical force to assemble — see the Tolerances page for fit calculations (H7/p6, H7/s6, etc.).

Shrink fit: heat the hub (or cool the shaft) for thermal expansion clearance during assembly, then it locks by interference at room temperature — see the Stress page for the ΔT calculator.

Advantage: zero backlash, no stress concentration from a keyway.
Disadvantage: not easily disassembled; requires tight tolerance control.

Single-plate (automotive style): simple, compact, moderate torque.

Multi-plate (wet or dry): multiple friction surfaces in the same envelope → much higher torque capacity for the same diameter. Wet (oil-bathed) runs cooler, longer life; dry is simpler but generates more heat.

Cone clutch: self-energizing wedge action — needs less axial force than a flat plate for the same torque, but harder to disengage cleanly.

Centrifugal clutch: engages automatically above a threshold RPM (chainsaws, go-karts) — no external actuation needed.

Shear pin coupling: a sacrificial pin shears at a set torque — simple, cheap, but must be replaced after every trip (and machine stops completely).

Slip clutch (friction): slips continuously above set torque, resets automatically once overload clears — no downtime for replacement, adjustable torque setting.

Ball detent torque limiter: balls pop out of detents above set torque, can be reset by reversing briefly — good compromise between shear pin and slip clutch.

Where used: conveyor drives, agricultural PTOs, robotics (collision protection), any drivetrain where a jam must not destroy the motor/gearbox.

Disc brake: high heat dissipation, consistent friction, easy inspection — standard for dynamic (moving) braking.

Drum brake: self-energizing (some designs), more torque per unit actuation force, but heat buildup limits duty cycle.

Electromagnetic brake: engages/releases via coil current — power-off engaged (fail-safe) versions are common for elevators, hoists, safety-critical holding brakes.

Regenerative braking: motor acts as generator, converts kinetic energy to electrical — used with VFDs/servo drives, reduces mechanical brake wear.

The most common clutch/brake sizing mistake: sizing for torque alone and ignoring thermal capacity.

Energy per engagement: E = ½×I×ω² (rotational kinetic energy to be absorbed), where I = moment of inertia, ω = angular velocity.

Duty cycle matters: a clutch rated for occasional engagement will overheat and glaze/fail if cycled continuously at high frequency — always check the manufacturer's thermal duty-cycle rating, not just static torque rating.