NMRV Worm Gearbox: Aluminum Housing Sizes RV25 to RV150
An NMRV worm gear reducer is a single-stage worm drive built into a die-cast aluminium housing, made in ten frame sizes named after the worm centre distance: RV25 through RV150. The number in the designation is the centre distance in millimetres, and once you know that, most of the selection process becomes a matter of matching torque, ratio and flange to the right step on that ladder.
This guide covers what the NMRV designation actually says, why the aluminium housing is a design choice rather than a cost saving, how the size ladder climbs from RV25 to RV150, and where the series fits against cast-iron worm alternatives. It closes with a short selection sequence and the questions buyers ask most.
What the NMRV Designation Means
The designation is compact and worth decoding, because it tells you the architecture before any drawing is opened.
- N - the series family code for this aluminium worm gear unit.
- MRV - the worm-gear layout itself: a cylindrical worm meshing with a bronze worm wheel on an output shaft set at a right angle to the input.
- The number - the centre distance between the worm axis and the output shaft axis, in millimetres. RV25 has a 25 mm centre distance; RV150 has 150 mm.
The centre distance is the honest measure of the frame. Everything that carries load - the worm diameter, the wheel diameter, the bearing spacing, the housing section - scales with it, which is why two NMRV units with the same ratio but different sizes behave so differently. A 30:1 at RV30 and a 30:1 at RV150 deliver the same speed reduction; the torque they can sustain continuously differs by roughly an order of magnitude.
The series is also known by close cousins - NRV for builds without a motor flange machined on the input, and NMRV with the flange ready for direct motor mounting. The gearing inside is identical.
Why Aluminium, Not Cast Iron
Traditional worm reducers - the WP and CWU families among them - wear grey cast-iron housings. The NMRV series deliberately does not, and the reasons are practical rather than cosmetic.
Weight is the first. An aluminium die-cast housing of the same stiffness is a fraction of the mass of cast iron, and worm reducers are frequently mounted on machine frames, brackets and small conveyors where every kilogram on the overhang matters. A fitter can lift the smaller frames by hand; even an RV130 or RV150 remains manageable for two people without dedicated lifting gear.
Corrosion resistance is the second. Aluminium does not rust the way bare cast iron does, which matters in food plants, wash-down areas, humid warehouses and outdoor installations. Paint adheres well when a specific colour or a food-grade finish is required, but many operators run the housings bare for decades.
Heat dissipation is the third, and it is the subtle one. A worm drive converts more of its input into heat than a helical train does, because the worm slides against the wheel teeth. The die-cast aluminium body, often with cooling ribs cast into its surface, sheds that heat to the surrounding air far better than a thick iron wall does. For a given frame size, that means a higher continuous duty rating before the oil temperature climbs into the limit band - and it is one reason the series is so common on machines that run all day.
| Property | Aluminium die-cast housing (NMRV) | Cast iron housing (WP / CWU types) |
|---|---|---|
| Weight at equal size | Much lower; smaller frames lift by hand | Several times heavier |
| Corrosion behaviour | Resists rust; suits washdown and outdoor use | Needs coating or paint to survive damp sites |
| Heat rejection | Good; ribs shed worm-sliding heat quickly | Slower; relies on wall thickness and surface area |
| Vibration damping | Moderate | Excellent; iron deadens noise well |
| Typical duty emphasis | Light-to-medium continuous duty, machine-integrated | Heavy duty, plant-floor installations |
| Cost at small sizes | Low; casting suits high volumes | Higher tooling and machining content |
The trade-off is real: iron damps vibration better and carries heavier shock duty, which is why cast-iron families still dominate the plant-floor end of the market. For machine-integrated drives up to moderate torque, aluminium usually wins the comparison outright.
The RV25 to RV150 Size Ladder
Ten sizes cover the range, and the ladder logic is worth internalising because it governs every downstream choice.
| Size | Centre distance | Single-stage ratio band | Typical duty class |
|---|---|---|---|
| RV25 | 25 mm | 5:1 to 100:1 | Light machines, dosing, small conveyors |
| RV30 | 30 mm | 5:1 to 100:1 | Small packaging and labelling machinery |
| RV40 | 40 mm | 5:1 to 100:1 | Light material handling, stirrers |
| RV50 | 50 mm | 5:1 to 100:1 | Medium conveyors, mixers |
| RV63 | 63 mm | 5:1 to 100:1 | Continuous-duty packaging lines |
| RV75 | 75 mm | 5:1 to 100:1 | Heavier conveyors, agitators |
| RV90 | 90 mm | 5:1 to 100:1 | Process machinery, screw feeds |
| RV110 | 110 mm | 5:1 to 100:1 | Medium industrial duty |
| RV130 | 130 mm | 5:1 to 100:1 | Heavy continuous conveyor duty |
| RV150 | 150 mm | 5:1 to 100:1 | Top of the series; high-torque slow shafts |
Two things stand out. First, the ratio band is the same across the range - roughly 5:1 to 100:1 in a single stage, with preferred ratios at 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 80 and 100. Size never buys ratio; it buys torque. Second, because the band tops out around 100:1, duties needing more than that combine a worm unit with a helical or bevel input stage, or move to a different architecture entirely.
The same size logic runs in reverse for small machines: an RV25 or RV30 replaces a belt drive or a chain-and-sprocket reduction with a sealed, rated unit that fits inside a machine cover - one reason the series is ubiquitous in packaging, food processing and logistics equipment.
Motor Flanges and Output Arrangements
The NMRV series is designed to swallow standard motors directly, and the input options matter as much as the ratio when a machine is being laid out.
The machined input accepts IEC motor flanges. A motor of the right flange bolts straight onto the reducer, the motor shaft seats into the flexible coupling inside the input, and the drive arrives as one sealed unit with no baseplate, no coupling guard and no alignment session. NEMA flange adapters cover the imperial motor world for export machines.
The output side offers the same modularity. A solid keyed shaft is the default; a hollow bore with a shrink disc takes a driven shaft straight through; an output flange bolts the reducer onto a machine shaft or a driven roll; and torque-arm arrangements suit pin mounting on small conveyors. Because the output is at a right angle to the motor, the whole drive turns a corner inside one compact casting - frequently the reason a machine designer chooses a worm layout at all.
Mounting position flexibility is a genuine convenience of the series: the units are filled for life with synthetic oil in the standard build, and most sizes can be installed in any orientation without re-specifying the lubrication or the breather.
Choosing a Size Without Guesswork
A short sequence settles the frame:
- Fix the output speed and the ratio. Motor speed divided by required output speed gives the ratio; check it sits inside the single-stage band, otherwise plan a combined unit.
- Compute the output torque, including the service factor for the duty class. Starting conditions and the number of starts per hour belong in the same arithmetic.
- Take the smallest frame whose continuous torque rating covers the factored requirement with margin - the aluminium body's heat rejection helps here, but the rating table is still the deciding document.
- Confirm the interfaces: IEC flange size for the motor you already use, and the output build the machine actually needs.
Running the numbers takes minutes in the online selection system, and it produces the duty point a supplier needs for a firm offer. For duties where the ratio sits at the deep end and the shaft carries a real overhung load, a helical-worm unit is often the better-balanced answer - the S67 helical worm gearbox is the typical mid-size example - and it is worth comparing both architectures before ordering.
For the aluminium worm series itself, the NMRV/NRV 130 worm gear reducer page shows the top-of-range frame with its standard builds. Send the duty point from the selector along with the motor data, and the frame, ratio and flange can be confirmed in one exchange.
Frequently Asked Questions
What does the number in an NMRV designation actually measure?
It is the centre distance between the worm axis and the output shaft axis, in millimetres. RV63 has a 63 mm centre distance. Everything load-bearing scales with that dimension, so it is the single most honest indicator of the frame's capacity.
Can an NMRV worm gear reducer take a standard motor directly?
Yes - that is one of the series' core design points. The input is machined for standard IEC flanges, so a matching motor bolts on without a baseplate, external coupling or alignment work. NEMA adapters cover imperial motors.
Is an NMRV unit self-locking?
At high ratios, a worm drive can approach self-locking behaviour, but it should never be relied on as a brake. Backdriving resistance depends on ratio, friction, lubricant and temperature, and vibration can defeat it. Any suspended or inclined load still needs a proper brake or a holding device.
Why choose aluminium over a cast-iron worm gearbox?
Weight, corrosion behaviour and heat rejection. An aluminium housing is much lighter for the machine frame to carry, does not rust in damp or washed-down environments, and sheds the worm drive's sliding heat quickly enough to raise the continuous duty rating of a given size. Cast iron keeps the edge in vibration damping and heavy shock duty.
How do I move up the RV size ladder correctly?
By factored torque, not by feel. Compute output torque from absorbed power and speed, apply the service factor for the duty class including starts, and select the smallest frame whose rating table covers the result. The ratio stays in the same band across all sizes - only the torque capacity climbs as the centre distance grows.
Send the duty point and motor details, and the correct frame from RV25 to RV150 - or a comparison against a helical-worm alternative - comes back as a firm, checked quotation rather than a guess.
