Mechanical efficiency:
how much of the power you supply does useful work

What an efficiency figure actually tells you

No real machine turns all the energy it receives into useful work. Some goes into friction in the bearings, some into heat in the windings, some into noise and vibration. Efficiency is the fraction that survives as the useful part.

Because it is a ratio of like quantities, efficiency has no unit and is normally written as a percentage. It can be defined for power (watts) or for energy (joules); the percentage is the same if both are measured over the same period.

An efficiency can never exceed 100 percent. A calculation that returns more than that is a sign of mixed units, a wrong time basis, or measuring input and output at different operating points.

Formula, with the loss alongside

Input and output must be in the same unit. A motor rated in horsepower and a load in watts both need converting; 1 hp is about 746 W for mechanical output.

Efficiency also depends on the operating point. Electric motors are often at their best at 60 to 100 percent of rated load and much poorer when lightly loaded, so one catalogue figure may not describe your use.

Worked example: 1000 W in, 750 W out

A small winch motor draws 1000 W from its supply and, measured at the drum, delivers 750 W of lifting power. Find its efficiency and the power lost.

Over an hour that 250 W loss is 0.25 kWh of wasted energy. At a price of, say, ₹8 per kWh (check your own tariff) it is about ₹2 an hour; small per hour, but significant for a machine that runs all day every day.

Cross-check with work and time: if the winch lifts a load by doing 2,700 kJ of useful work in an hour, the average output is 2,700,000 ÷ 3,600 = 750 W, matching the figure above.

Chaining machines together

When one machine drives another, the efficiencies multiply, not add. A motor that is 90 percent efficient driving a gearbox that is 80 percent efficient delivers 0.90 × 0.80 = 0.72 of the electrical input as useful output. Each extra stage is a fresh opportunity to lose a slice.

The overall figure is about 68 percent, even though no single component is below 80 percent. Reducing the number of stages is often more effective than polishing any one of them.

Typical ranges to sanity-check a result

• Large electric motors: often 90 to 96 percent at good load.
• Small induction motors: roughly 60 to 85 percent.
• Spur gear pairs: 95 to 99 percent per mesh; worm gears much lower, sometimes 40 to 70 percent.
• Petrol engines: about 25 to 35 percent in converting fuel energy to shaft work.
• Hydraulic pumps: commonly 80 to 90 percent when well matched.

These bands are broad because designs, sizes and conditions differ. Use them only as a plausibility check, and rely on the manufacturer's data sheet for your own machine.

Efficiency worked out from a lifting job

Often you know loads and distances rather than power. Efficiency can be calculated from the work done on the load compared with the work put in at the handle or motor.

A hoist raises a 50 kg load through 6 m in 4 s. Useful work is weight × height = 50 × 9.81 × 6 = 2,943 J, so the useful output power is 2,943 ÷ 4 = 735.75 W. If the motor draws 1,000 W during the lift, efficiency is 735.75 ÷ 1,000, or about 73.6%, and roughly 264 W goes to losses.

For simple machines such as pulleys and levers, the same idea gives η = (load × load distance) ÷ (effort × effort distance). A pulley system lifts a 1,000 N load by 1.2 m when a 300 N effort moves 5 m. Work out is 1,200 J and work in is 1,500 J, so the efficiency is 80 percent.

• Efficiency below 100 percent is guaranteed by friction; ideal machines are only a reference point.
• Higher mechanical advantage is not the same as higher efficiency.
• Measure input at the same stage in every comparison, or the percentages cannot be compared.

Where the lost power usually goes

When a figure comes out low, the loss normally concentrates in a few places. Look at the stage with the lowest efficiency first, because it is multiplied into the whole chain.

• Friction in bearings and seals: fix with the right lubricant and alignment.
• Belt slip and chain stretch: restore tension and replace worn parts.
• Operating far from the rated load: size the motor or pump to run near its best-efficiency point.
• Fluid losses in pipework: reduce sharp bends and undersized pipe.

A recorded before-and-after power measurement tells you whether a change helped. Even a gain of five percentage points on a machine that runs for thousands of hours a year pays back through lower energy use.

Pitfalls when working out efficiency

• Comparing power with energy. If input is in kWh and output in watts, convert them to the same basis first.
• Using nameplate power as actual input. A motor's rating is its maximum, not what it is drawing.
• Counting the wrong output. Useful output is what reaches the load, not the shaft power before the transmission.
• Mixing mechanical efficiency with overall efficiency. An electric motor's electrical-to-shaft figure is a different stage from shaft-to-load.

A mechanical work calculation helps with the output side when you know force and distance rather than power: work divided by time gives the average power to put in the numerator.