Worm gearbox efficiency data is scattered across catalog footnotes, ISO standards, and manufacturer test reports — rarely consolidated in a form engineers can bookmark and use directly in motor-sizing and energy-cost calculations. This article presents a single authoritative efficiency table covering every standard ratio from 5:1 to 100:1 under four real-world conditions (mineral oil vs PAO synthetic, cold start vs run-in steady state), the exact formula behind each data point, and a worked step-by-step calculation demonstrating how to apply the table to a real application. Whether you need to verify a catalog efficiency claim, size a motor correctly, or calculate annual energy cost, this is the reference page.
The Efficiency Formula — Understanding the Physics First
Before reading the table, understand the equation that generates every number in it. The ISO 14521 worm gear load capacity standard specifies worm gear efficiency as:
Where the variables mean:
- αn = normal pressure angle at the tooth surface, typically 20° for standard worm gears. cos 20° = 0.940.
- µ = coefficient of friction between the worm thread and the worm wheel tooth face. This is the most variable parameter — it depends on sliding speed, lubricant film thickness, surface finish, and material pair. Typical values: mineral oil 0.10–0.14; PAO synthetic 0.08–0.11; run-in units 0.08–0.10; new units 0.12–0.15.
- γ = lead angle of the worm — the angle between the worm thread helix and a plane perpendicular to the worm axis. Lead angle is directly related to reduction ratio: higher ratio → smaller lead angle → lower efficiency. This is the single dominant variable in the efficiency equation.
The lead angle for a single-start worm is approximately:
where i = ratio, d₁ = worm pitch diameter, a = center distance
For standard NMRV proportions (which are well-characterized), a useful approximation is that the lead angle in degrees ≈ arctan(π × d₁ / (i × π × m × z₂)), where m is the module and z₂ is the wheel tooth count. The table below uses representative lead angles derived from NMRV standard proportions as documented in ISO 14521.
The Master Efficiency Table — 5:1 to 100:1, Four Conditions
This table consolidates data from ISO 14521 methodology, experimental results published by the Gear Research Centre (FZG Munich), and aggregated manufacturer acceptance test data. Four columns capture the main variation bands seen in practice:
| Ratio (i) | Lead Angle γ° | Mineral Oil Run-in, 40°C |
Mineral Oil New unit / cold |
PAO Synthetic Run-in, 40°C |
PAO Synthetic New unit / cold |
Self-Lock |
|---|---|---|---|---|---|---|
| 5:1 | 21–26° | 86–90% | 80–84% | 89–93% | 83–87% | No |
| 7.5:1 | 18–22° | 83–88% | 77–82% | 87–91% | 81–86% | No |
| 10:1 | 14–18° | 80–85% | 74–79% | 84–88% | 78–83% | No |
| 15:1 | 10–14° | 75–80% | 69–74% | 79–84% | 73–78% | Borderline |
| 20:1 | 8–11° | 72–78% | 66–72% | 76–82% | 70–76% | Borderline |
| 25:1 | 7–9° | 70–76% | 64–70% | 74–80% | 68–74% | Usually yes |
| 30:1 | 6–8° | 68–74% | 62–68% | 73–79% | 67–73% | Yes |
| 40:1 | 5–7° | 65–71% | 59–65% | 70–76% | 64–70% | Yes |
| 50:1 | 4–6° | 64–70% | 58–64% | 69–75% | 63–69% | Yes |
| 60:1 | 3.5–5° | 62–68% | 56–62% | 67–73% | 61–67% | Yes |
| 80:1 | 2.8–4° | 58–64% | 52–58% | 63–69% | 57–63% | Yes |
| 100:1 | 2.3–3.5° | 55–61% | 49–55% | 60–67% | 54–61% | Yes |
Source: ISO 14521 worm gear load capacity methodology; FZG Munich experimental data; aggregated manufacturer EOL test records. “Run-in” = after 50–200 h at graduated load; “New/cold” = first cold start below 25°C oil temperature. Values represent 60–80% rated load — peak efficiency operating range. At <20% and >100% rated load, expect 2–5% lower efficiency.
How to Use the Table — Step-by-Step Worked Example
Scenario: An NMRV063 worm gearbox at 50:1 ratio drives a conveyor screw running 6,000 hours/year. The required output torque is 140 Nm. Motor runs at 1,400 rpm. Lubricant: PAO synthetic, run-in unit at 40°C operating temperature.
- Read efficiency from table: 50:1 ratio, PAO synthetic, run-in → η = 69–75%. Use midpoint 72% for motor-sizing; use lower bound 69% for thermal-budget worst-case.
- Calculate output power: Pout = T × ω = 140 Nm × (1400/50 × 2π/60) = 140 × 2.93 = 410 W output.
- Calculate required input power: Pin = Pout / η = 410 / 0.72 = 569 W. A 0.55 kW (550 W) motor is marginally undersized; specify 0.75 kW for service-factor margin.
- Calculate heat generation: Ploss = Pin − Pout = 569 − 410 = 159 W (worst case at η = 69%: 569 × 0.31 = 176 W). Verify the NMRV063 at 50:1 thermal input-power rating exceeds 0.569 kW — this is a modest load and any NMRV063 will clear this easily.
- Calculate annual energy waste: Ploss × hours = 0.159 kW × 6,000 h = 954 kWh/year wasted as heat.
- Calculate annual energy cost of losses: 954 × €0.12 = €114/year per drive in wasted energy.
For motor-sizing decisions referencing the full NMRV worm gearbox range, always confirm the thermal input-power rating from the specific catalog page — the thermal limit (not the mechanical limit) is often the binding constraint at ratios above 40:1 for continuous-duty applications.
How to Read the Table for Different Conditions
The four columns represent the major real-world efficiency variation bands. Use the correct column for your situation:
| Application Condition | Use This Column | Why |
|---|---|---|
| Motor sizing (continuous steady-state duty) | PAO Synthetic, Run-in | Best represents steady-state operating condition; use for Pin calculation |
| Thermal budget (worst-case heat generation) | Mineral Oil, New/Cold | Lowest efficiency = maximum heat; use lower bound for thermal safety margin |
| Annual energy cost calculation | PAO Synthetic, Run-in | Energy cost accumulates at steady-state operating condition, not cold-start |
| Startup / cold-start torque estimation | Mineral Oil, New/Cold | Cold lubricant produces highest friction — use for e-stop torque and starting current |
| Comparing supplier catalog claims | PAO Synthetic, Run-in | Catalog efficiency values are always measured run-in at rated temperature |
Efficiency vs Load Level — The Often-Missed Dimension
The table above gives efficiency at 60–80% rated load — the range where worm gearboxes operate most efficiently. Efficiency varies significantly with load level, and many engineers miss this:
- At <20% rated load: No-load losses (bearing drag, seal friction, oil churning) are a large fraction of the total power flow. Efficiency drops 3–7 percentage points below the table values. A gearbox running at 10% load often operates at only 50–55% efficiency even at low ratios.
- At 60–80% rated load: Peak efficiency range. Table values apply directly.
- At 90–100% rated load: Mesh contact pressure increases slightly, raising friction. Efficiency drops 1–3 percentage points below peak.
- At >100% rated load (overload): Efficiency drops sharply, heat generation spikes, and the risk of lubricant film breakdown increases rapidly. Never rely on efficiency table values at overload conditions.
For applications where the gearbox consistently runs below 40% rated load (very light-duty intermittent applications), the actual efficiency in service will be 5–10 percentage points below what the catalog or this table predicts at rated conditions. If energy cost matters for these applications, specify the next smaller frame size running at 60–70% of its rating rather than an oversized unit running at 20% of its rating — the smaller unit will deliver both better efficiency and lower purchase cost.
Multi-Start Worms — How Ratio and Efficiency Interact
The efficiency table above applies to single-start worm gearboxes — the most common commercial configuration. Multi-start worms (2-start, 3-start, 4-start) change the lead angle geometry and consequently the efficiency profile:
| Worm Starts | Example: 60-tooth wheel | Ratio | Lead Angle | Eff. (PAO, run-in) | Self-Lock |
|---|---|---|---|---|---|
| 1-start | 1/60 turns per rev | 60:1 | ~4° | 67–73% | Yes |
| 2-start | 2/60 turns per rev | 30:1 | ~8° | 73–79% | Yes |
| 4-start | 4/60 turns per rev | 15:1 | ~16° | 82–87% | No |
This table illustrates the S-Series helical-worm design principle: splitting the total ratio across a helical pre-stage and a lower-ratio worm final stage effectively gives the worm stage a larger lead angle at the same overall ratio — delivering the efficiency of a 2-start or 4-start worm while retaining a single-start self-locking worm final stage. For the full technical background, see the worm gearbox technical selection guide.
Frequently Asked Questions
Why do catalog efficiency values differ from my measured values?
Catalog efficiency values are measured on run-in units at rated load and 40°C oil temperature — the most favorable conditions. In the field, new gearboxes may run 4–8% less efficiently before run-in is complete; units running below 40% rated load may run 5–8% less efficiently than catalog values; units in cold or hot ambients may run outside the catalog temperature assumption. If your measured efficiency is 5–10% below catalog, check these three conditions before assuming a faulty unit.
Can I use these efficiency values for double-stage (double-reduction) worm gearboxes?
Yes — but you must apply them per stage. For a 400:1 double-reduction unit (20:1 × 20:1): read 20:1 PAO run-in efficiency from the table (76–82%), then multiply: 0.79 × 0.79 = 62% combined efficiency (midpoint estimate). Always use the individual stage ratio — not the total ratio — when looking up efficiency in this table for multi-stage configurations.
Does efficiency change at higher or lower input speeds?
Yes — but modestly within the typical 700–2,800 rpm input range. Higher input speed increases sliding velocity, improving lubricant film thickness and slightly reducing the friction coefficient, which improves efficiency by 2–4 percentage points. Very high speeds (above 3,000 rpm) introduce churning losses that offset this gain. The table values are specified at 1,400 rpm — the most common 4-pole motor speed. For 2-pole motors (2,800 rpm), expect approximately 2% better efficiency; for 6-pole motors (950 rpm), expect approximately 2% worse.
How do I verify efficiency if I only have an input current meter and output speed?
Practical method: measure (1) motor input kW via power meter, (2) output torque via torque wrench on the driven load or via current draw under known load, (3) output shaft speed via tachometer. Calculate η = (Tout × ωout) / Pin. If a torque wrench is not available, the indirect thermal method is simpler: measure steady-state housing temperature above ambient (ΔT). A gearbox at 70% efficiency dissipates 30% of input as heat; the housing ΔT above ambient is proportional to the heat generated per unit of surface area — compare to the manufacturer’s thermal rating to verify the gearbox is operating within spec.
Need a Worm Gearbox Sized for Your Specific Efficiency Requirement?
Share your ratio, load torque, operating hours, and temperature — we’ll confirm the efficiency at your exact operating point and provide a sized recommendation from our NMRV range.
Energy Cost Calculator — Apply the Table to Any Application
The three-step calculation below converts efficiency table data into annual energy cost — the number that makes a business case for lubricant upgrades, architecture changes, or fleet replacement programs:
- Step 1 — Find annual power waste: Ploss (kW) = Pinput × (1 − η). Read η from the table using your ratio and lubricant condition. Example: 5.5 kW input at 50:1, PAO run-in (η = 72%) → Ploss = 5.5 × 0.28 = 1.54 kW.
- Step 2 — Multiply by annual hours: Ewaste (kWh) = Ploss × h/year. Example: 1.54 kW × 5,000 h = 7,700 kWh/year.
- Step 3 — Apply energy tariff: Cost = Ewaste × tariff. Example: 7,700 × €0.12 = €924/year per drive in wasted energy.
Scale this to a fleet to reveal the business case for lubricant upgrades or efficiency improvements:
| Fleet Size | Annual Waste (Mineral Oil, 50:1) | Annual Waste (PAO, 50:1) | Saving by Switching to PAO |
|---|---|---|---|
| 1 drive (5.5 kW, 5,000 h/yr) | €1,056/yr | €924/yr | €132/yr |
| 10 drives | €10,560/yr | €9,240/yr | €1,320/yr |
| 50 drives | €52,800/yr | €46,200/yr | €6,600/yr |
For a 50-drive plant, switching from mineral oil to PAO synthetic saves €6,600/year in energy — a lubricant re-fill cost that pays back in 2–4 months at typical PAO lubricant cost premiums. The efficiency difference between the two lubricant columns in the master table (3–5 percentage points) is the quantitative basis for this payback calculation, and the reason PAO is almost universally recommended for continuous-duty worm gearbox applications running more than 6 hours per day.
Common Mistakes When Using Efficiency Data
- Using catalog efficiency for new cold-start motor sizing: Catalog efficiency (run-in, rated temperature) is the right value for steady-state motor sizing. For cold-start current calculations, use the New/Cold column — efficiency is 6–10% lower, meaning starting torque and current are higher than steady-state estimates would suggest.
- Ignoring load level: The table applies at 60–80% rated load. If your application consistently runs below 30% rated torque, the actual efficiency in service is 5–8% lower than table values. Reselect a smaller frame size and re-check.
- Using total ratio for double-stage units: For a 400:1 double-stage, do not look up 400:1 — instead look up the individual stage ratios (e.g., 20:1 each) and multiply: ηcombined = ηstage1 × ηstage2.
- Assuming efficiency is constant across the service life: Run-in units (50–200 hours) are 4–8% more efficient than new units. Worn units approaching end-of-life are 5–10% less efficient than run-in units. Track efficiency trend (via temperature rise monitoring) as a service-life indicator.