Power transmission components and alignment
Power transmission components and alignment questions cover belts, chains, gears and couplings, and the measurement and correction of shaft misalignment.
Gears and couplings
Module m = d/z, so a gear with z = 40 and m = 3 has d = 120 mm; with a z = 20 pinion the centre distance is a = (60 + 120)/2 = 90 mm. Backlash and the tooth contact pattern, shown by marking compound, reveal an incorrect centre distance or helix angle. Rigid couplings suit perfectly aligned short shafts; flexible couplings (elastomer, gear, grid) absorb misalignment and shock.
Belts and chains
V-belts are tensioned so the slack-side deflection is about 16 mm per metre of span under firm thumb pressure, and matched sets are replaced together - never one belt of four. Pulleys are aligned with a straight edge on both faces. Chains need roughly 2 % of span slack on the slack side, correct sprocket alignment and regular lubrication.
Alignment
Misalignment is parallel (offset) or angular (gap). Rim-and-face readings with a dial indicator, or a laser system, measure both; bracket sag and indicator resolution affect the result. Correct in two planes, checking for soft foot first by tightening each foot in turn with an indicator underneath. Align cold machines with a compensation for thermal growth.
Worked example
A face reading of 0.12 mm across a 100 mm coupling diameter is an angular misalignment of 0.12/100 = 1.2 mm/m. The front motor foot is 300 mm from the coupling centre, so the shim correction is 0.12 × 300/100 = 0.36 mm. The reasoning: the gap error scales linearly with distance from the pivot at the coupling centre, so a foot three diameters away needs three times the correction.
Traps
Aligning a hot machine to cold targets without thermal compensation, skipping the soft-foot check, mixing old and new belts in one drive, measuring chain slack on the tight side, and accepting an alignment as "close enough" without recording indicator values.
Workplace safety, environment and quality control
Workplace safety, environment and quality control questions test hazard recognition and controls, waste and spill handling, and the inspection, calibration and statistical tools that keep production inside specification.
Safety
Identify the hazard and control it in order: eliminate, guard, then use PPE as the last line - goggles for grinding and drilling, hearing protection above 80 dB(A), safety shoes. No gloves, loose sleeves or jewellery at rotating machines, because gloves can be drawn into a lathe. Guards and interlocks stay fitted, and the emergency stop is tested rather than used as a normal stop. Before maintenance, isolate and lock off energy (LOTO). Never use compressed air on skin or clothing. Fire classes: CO₂ for electrical equipment and flammable liquids, water for ordinary solids, foam for liquids, dry powder for gases - never water on live electrical equipment.
Environment
Coolant is hazardous waste: skim tramp oil, keep the sump clean and dispose through a licensed contractor, never to the drain. Segregate swarf, oily rags, batteries, WEEE and packaging; keep a spill kit and bunded storage for oils; read the safety data sheet before handling a chemical; extract welding fumes and solvent vapour.
Quality control
ISO 9001 provides the process framework. Incoming, in-process and final inspection follow the control plan, and instruments need current calibration traceable to national standards. On an X̄–R chart the control limits come from the process (±3σ), not from the drawing; the drawing limits are specification, not control. Cp = (USL − LSL)/(6σ) and Cpk = min[(USL − x̄), (x̄ − LSL)]/(3σ), with Cpk ≥ 1.33 as the usual minimum. Nonconformities are closed with root-cause analysis (5 Whys, 8D) and verified corrective action.
Worked example
A pin is specified 25.00 ± 0.05 mm. The process mean is 25.02 mm with σ = 0.012 mm. Cpk = min(25.05 − 25.02; 25.02 − 24.95)/(3 × 0.012) = min(0.83; 1.94) = 0.83. The process is off-centre and not capable. Centring the mean at 25.00 gives Cpk = 0.05/0.036 = 1.39, which passes.
Traps
Wearing gloves at a lathe, confusing control limits with tolerance limits, judging a process capable from Cp alone when it is off-centre, and inspecting with an instrument that has no valid calibration.