NMRV Worm Gearbox: Ratios, Sizes, Selection & What Drives the Price in India

A buyer's guide to NMRV worm gearboxes from an Ahmedabad manufacturer — 7.5:1 to 100:1 ratios, 0.06–15 kW, torque to 1,550 Nm, frame sizes, how to size one correctly, and exactly what to send us for an accurate quote.

Short answer: an NMRV worm gearbox is the right choice when you need a large speed reduction in a small, light, low-noise package and the duty is light to moderate. Super Mech Industries manufactures NMRV units in Ahmedabad with ratios from 7.5:1 to 100:1, a power range of 0.06 kW to 15 kW and output torque up to 1,550 Nm, in aluminium alloy housings with bronze worm wheels, filled for life. If your duty is continuous, high-shock or above ~15 kW, a helical or shaft-mounted unit is usually the better engineering answer — this guide covers both cases so you order the right thing the first time.

Most buying mistakes with worm gearboxes come from ordering by frame size alone ("send me an NMRV 63") without stating ratio, mounting or duty. That produces a unit that physically fits and then runs hot, or one that is two sizes larger than needed. The sections below give you the selection sequence we use on the shop floor.

What an NMRV gearbox actually is

NMRV is the industry designation for a compact worm-and-wheel reducer with an aluminium alloy housing. A hardened steel worm on the input shaft meshes with a bronze worm wheel on the output shaft, set at 90°. That right-angle layout is the reason it is so widely used: the motor sits perpendicular to the driven shaft, which saves a great deal of space on conveyors, packaging lines and textile machines.

The trade-off is efficiency. A worm mesh is a sliding contact, not a rolling one, so it loses more power as heat than a helical mesh does. Efficiency falls as the ratio rises — a low-ratio worm set is reasonably efficient, while a 100:1 unit converts a substantial share of input power into heat. That single fact drives most of the selection rules further down this page.

Where the NMRV wins

• Space-constrained drives — the right-angle output lets the motor tuck alongside the machine frame.
• High reduction in one stage — a single worm set does what would take two or three helical stages.
• Quiet running — sliding contact generates far less gear whine than a spur or helical train.
• Light structures — the aluminium housing weighs a fraction of a cast-iron unit, which matters on moving gantries and light frames.
• Corrosive or wash-down areas — the aluminium alloy body resists corrosion better than untreated cast iron.

NMRV specification range we manufacture

Full details and photographs are on the NMRV Gear Box product page. Exact per-frame torque, shaft diameters and mounting dimensions vary by size and ratio — ask us for the dimension sheet for the specific frame you are considering before you finalise a machine drawing.

Reading the frame size

The number in an NMRV designation is the centre distance in millimetres between the worm and the wheel. NMRV 40 has a 40 mm centre distance; NMRV 90 has 90 mm. It is a size code, not a rating. Two units with the same frame number can have very different output torques depending on ratio, so never order on frame number alone. The common frame progression runs 025, 030, 040, 050, 063, 075, 090, 110, 130 and 150, and larger frames carry proportionally more torque.

How to size an NMRV gearbox in four steps

Step 1 — fix the output speed, then find the ratio

Start from the machine, not the gearbox. If your conveyor needs to run at 48 RPM and you are driving it with a standard four-pole 1,440 RPM motor, the ratio you need is 1,440 ÷ 48 = 30:1. Work in that direction every time. Ordering a ratio first and hoping the speed lands right is how lines end up running 20% fast.

Step 2 — calculate the torque the gearbox must deliver

Output torque follows from power and output speed. The standard relation is:

T (Nm) = 9,550 × P (kW) ÷ n (RPM)

For a 2.2 kW motor turning an output shaft at 48 RPM, that is 9,550 × 2.2 ÷ 48 ≈ 438 Nm before losses. Compare that figure against the gearbox rating, not against the motor rating — a gearbox that suits 2.2 kW at 200 RPM is not the same unit that suits 2.2 kW at 20 RPM, because the torque at the slower output shaft is ten times higher.

Step 3 — apply a service factor

The calculated torque is the steady-state number. Real machines start, stop, jam and shock-load. Multiply by a service factor before you select:

If your application sits in the bottom row and runs continuously, stop and reconsider the technology. Worm gearboxes are not the natural choice for heavy continuous shock duty — that is where a shaft mounted speed reducer or a helical unit belongs.

Step 4 — settle mounting and shaft details

This is the step that most often causes a return. Decide and state clearly:

• Mounting — foot mounted, flange mounted, or hollow output shaft slipped straight onto the driven shaft.
• Output form — solid shaft with key, or hollow bore. If hollow, give the driven shaft diameter and keyway.
• Input — IEC motor flange (state the frame, e.g. B5/B14 and the motor frame size) or free input shaft for a belt or coupling drive.
• Orientation — which way the output shaft must point relative to the motor, since worm units are handed.

Thermal rating: the limit people forget

A worm gearbox has two limits — a mechanical torque rating and a thermal rating, which is the power it can shed as heat continuously without overheating the oil. At high ratios and long duty cycles the thermal limit bites first. A unit that is mechanically fine at 30 minutes on / 30 minutes off may cook if you run it 20 hours a day in a hot Gujarat summer shed with no air movement.

Practical rules that keep NMRV units alive:

• Leave clear air around the housing — the fins are the cooling system.
• Avoid boxing the gearbox into a sealed guard or a dust-packed corner.
• For continuous duty above roughly 50:1, tell us the running hours so we can check the thermal case, not just the torque case.
• Surface temperature that is uncomfortable to touch for more than a second is a warning sign, not normal operation.

Worm or helical? An honest comparison

Our position as a manufacturer: below about 5 kW, in intermittent or moderate duty, with a right-angle layout needed, the NMRV is usually the sensible and economical answer. Above that, or in continuous heavy duty, look at a helical geared motor or a gear motor. We make all three, so we have no reason to push you toward the wrong one.

What actually drives the price of an NMRV gearbox in India

Buyers often ask for "the NMRV price list". There isn't one meaningful number, because the same frame size can vary substantially in cost. The real drivers are:

• Frame size — the dominant factor. Material and machining scale with centre distance.
• Ratio — high-ratio worm sets need more precise wheel cutting and a larger bronze wheel blank.
• Bronze content and grade of the worm wheel. This is where cheap units cut corners, and it is invisible until the wheel wears out early. A worm wheel with insufficient bronze quality will show tooth wear within months on a continuous line.
• Motor integration — a bare gearbox, an IEC-flanged unit and a complete motorised gear unit are three different prices.
• Output configuration — hollow shaft, double output shaft, torque arm or special bore all add machining.
• Quantity — batch sizes change setup cost per unit meaningfully.
• Special requirements — non-standard bore, painted finish, food-grade oil, shaft materials, test certificates.

A quote that arrives without any of these questions being asked is a quote for something generic. We would rather ask six questions and send you a number you can actually order against.

Common failures and what caused them

• Bronze wheel worn in months — almost always undersized for the duty, or run past its thermal limit, or a poor-grade wheel. Check the service factor calculation first.
• Oil leaking at the output seal — usually an overhung load problem. A belt pulley or sprocket hanging off the output shaft applies a side load the bearing was not sized for. Tell us the pulley diameter and belt tension and we will size for it.
• Housing too hot to touch — thermal limit, poor ventilation, or the wrong ratio for continuous duty.
• Unit back-drives under load — the self-locking assumption. Add a brake; do not rely on the worm.
• Noise and vibration after a bearing change — worm sets are sensitive to preload and backlash. Rebuilds need the right shim stack, not a best guess.

What to send us for an accurate quote

Send these seven items and we can size, price and give you a delivery date in one reply rather than three:

1. Motor power in kW and speed in RPM (or state that you need the motor from us).
2. Required output speed in RPM, or the ratio you have already selected.
3. Required output torque in Nm, if you know it.
4. Mounting: foot, flange or hollow shaft — and the output orientation you need.
5. Output shaft diameter or driven shaft bore, with keyway.
6. The driven machine and its duty — hours per day, starts per hour, shock or smooth.
7. Quantity, and where it ships to.

If you have a drawing, an old nameplate photograph or a sample unit, that is faster still — send a photograph of the existing gearbox nameplate and we can usually match it directly.

Why buy from Super Mech Industries

We are an ISO 9001:2015 certified manufacturer based in Ahmedabad, Gujarat, making gear units, couplings, brakes, pulleys and sprockets under one roof, and exporting to 29 countries. That matters for a gearbox order in three practical ways: bores and shafts are machined to your drawing rather than to a catalogue standard, a matching coupling, pulley or brake can ship with the same order, and if a unit needs to be looked at, it comes back to the people who made it.

For the drive side of the same machine, see our gear coupling range and the torque calculator — the same torque figure you calculate above is the input for coupling selection.

Frequently asked questions

What ratios and power ratings are available in an NMRV worm gearbox?

We supply NMRV units with ratios from 7.5:1 to 100:1, power from 0.06 kW to 15 kW, and output torque up to 1,550 Nm, in aluminium alloy housings with bronze worm wheels, filled for life with synthetic oil.

What does the number in NMRV 63 mean?

It is the centre distance in millimetres between the input worm and the output wheel. It is a frame-size code, not a ratio or a torque rating — two NMRV 63 units can have very different ratios and output torques.

Is an NMRV worm gearbox self-locking?

Do not rely on it. High-ratio worm sets resist back-driving under static load, but self-locking is not a safety function — vibration, shock and temperature all reduce the effect. For lifting or hold-in-position duty, specify a motor brake or an electromagnetic disc brake as the holding device.

How do I calculate the output speed?

Divide motor speed by the ratio. A 1,440 RPM motor on a 30:1 unit gives 48 RPM at the output shaft. Work backwards from the speed your machine needs to find the ratio to order.

Can you supply the gearbox with a motor fitted?

Yes. We supply bare gearboxes, IEC-flanged units and complete motorised gear units. Tell us the voltage and phase and whether you need a brake motor.

Do you deliver outside Gujarat?

Yes — we despatch across India from Ahmedabad and export to 29 countries. Give us the delivery pincode or port with your enquiry and we will include freight and lead time in the quote.

Super Mech Industries — Odhav, Ahmedabad, Gujarat 382415, India. Phone: +91 63510 70577. Email: smigvs@gmail.com. ISO 9001:2015 certified.