The R series helical gear reducer is a coaxial, foot-mounted unit in which the motor and the driven shaft share one straight centreline, with two or three stages of ground helical gearing between them. That single geometric fact drives everything else: high efficiency per stage, low noise, a long, narrow footprint, and a shaft arrangement that suits conveyors, mixers, pumps and any machine where the motor can sit directly behind the drive. This guide covers how the designation is built, how to size the unit from torque rather than from motor power alone, what the mounting and lubrication rules are, and when the inline layout stops being the right answer.

What "Inline" Actually Buys You

In a coaxial unit the torque path never turns a corner. Every stage is a pair of parallel-axis helical gears, so the power flows straight from the input shaft to the output shaft along one axis. Compared with a right-angle build, that removes the bevel pair and the extra bearing set that carries it, which shows up in three places: efficiency, noise and axial length.

Efficiency per helical mesh typically sits in the high nineties, so a two-stage unit retains more of the motor's output than an equivalent right-angle train, and noise drops because helical teeth engage gradually along a slanted contact line instead of all at once.

The trade is packaging. A coaxial unit is long and the motor must be reachable along that same line. At a conveyor head that is no problem; where the motor has to tuck under a deck or beside a tank wall, the right-angle layout earns its extra loss.

Reading the Frame Code

The designation is a compressed drawing specification: every character constrains a dimension or a machining operation, which is why a quotation built on ratio alone will not survive contact with the installation drawing.

Frame numbers do not step evenly in torque. Each size is designed around a band and the top of one frame often overlaps the bottom of the next, so a duty near a boundary may pass on thermal rating in the smaller frame while passing on peak torque in the larger one. The choice then becomes a question of duty cycle rather than catalogue arithmetic.

Where a plant runs several drives of the same series, staying inside one or two frame sizes keeps spares, oil grades and fitting skills in one column - the reason a weekend breakdown can be solved with a part that is actually on the shelf.

Sizing From Torque, Not From Motor Power

The commonest sizing error is to choose the frame from the motor nameplate. Motor power tells you what the drive can deliver, not what the machine demands. The starting point is output torque at the driven shaft, with the speed and duty profile.

Output torque follows directly from power and speed:

T (N·m) = 9550 × P (kW) / n (rpm)

A 7.5 kW motor at 1450 rpm develops roughly 49 N·m at the motor shaft. Through a 40:1 reduction that becomes about 1980 N·m at the output, minus the gearing losses. The frame must then be checked against that torque multiplied by the service factor the application demands, not against the raw figure.

Two further checks decide whether the frame is genuinely adequate. Thermal capacity asks whether the housing can dissipate the heat its losses generate at the actual ambient temperature and duty cycle; a unit that passes on torque can still run hot in a badly ventilated enclosure. Overhung load asks whether the radial force at the output shaft, from a sprocket, pulley or heavy coupling, stays inside the bearing rating at the stated distance from the shoulder. Omitting the overhung load is the one omission that consistently forces a redesign, because at medium sizes the output bearing runs out of margin before the gear teeth do.

For duties that sit close to a boundary in several parameters at once, the online selector at the WGT selection tool lets you sweep frame and ratio combinations against torque, speed and mounting before an enquiry is written.

Ratio, Stage Count and the Speed You Actually Get

A coaxial unit reaches its ratio in two or three stages, and the stage count is not free: each additional mesh adds losses, length and a shaft. Two-stage builds typically cover the moderate range; three-stage builds cover deep reductions where a single pair would need an impractically small pinion.

Motor pole count sets the input speed and often matters more than the ratio. A four-pole motor at about 1450 rpm and a six-pole motor at about 960 rpm can reach the same output speed by different routes, and the six-pole route usually lands in a gentler stage count at the cost of a larger motor.

Check the delivered output speed rather than the nominal ratio, because slip moves the number. A conveyor tolerates a percent or two; a machine indexed to a fixed cycle may not. Write the output speed and its tolerance into the enquiry so the ratio can be trimmed at the drawing stage.

Mounting Rules That Decide Service Life

A foot-mounted coaxial unit is rigid in the wrong directions if it is bolted to something that moves. The rules that matter are simple but frequently skipped.
- Flat, machined and stiff. The base must be flat to within a fraction of a millimetre across the foot length and stiff enough not to deflect under torque reaction. Shimming a flexible frame with washers is a reliable way to crack a housing foot.
- Alignment comes from machining. The motor couples directly to the input shaft, so any angular or parallel misalignment is taken by the coupling - a flexible one absorbs a little, a rigid one absorbs none.
- Torque reaction belongs to the feet. A foot-mounted unit transmits reaction through its base bolts, so the anchor design deserves the attention of the drive itself.
- Leave the breather where it belongs. Fit it in the position stamped for the installed posture, and never plug it to stop a weep.

Lubrication and the First Year of Service

Helical units are typically splash-lubricated with mineral or synthetic gear oil chosen by ambient temperature and by the oil chart for the mounting posture. Read the level at standstill with the unit level; checking while the shaft turns is the classic route to overfilling, which pushes oil past the seals.

The first oil change matters more than any later one, because running-in releases fine wear particles from the new gear surfaces. Take the opportunity to look at what drains out: a magnetic plug with a modest fuzz is normal, metallic flakes or a gritty sheen are worth a conversation with the supplier.

Vibration and temperature are the two cheap indicators. A monthly handheld reading taken at the same point and load builds a baseline within a few visits, and a housing running warmer than its neighbours is reporting something long before it fails.

Where Inline Stops Being the Answer

The coaxial layout loses when the motor cannot sit on the machine's axis: tank tops where the process shaft points down, pump decks with no headroom for a motor tower, drive ends already occupied by a handwheel or cabinet, and shafts so long that a coupling line beneath the machine becomes a maintenance liability.

In those cases a right-angle first stage pays for itself. The extra bevel pair costs a few percent of efficiency, but it moves the motor to a walkway and shortens the drive train. A vertical process axis also changes the seal and bearing duty, because the shaft below the unit generates thrust and radial whip a horizontal drive never sees.

Matching the layout to the machine is the whole of the decision. A right-angle unit forced into a straight-line application buys nothing but loss.

A Short Enquiry Checklist

An enquiry that gets a correct answer in one exchange carries: output torque or motor power with service factor, required output speed and tolerance, mounting posture and interface dimensions, overhung load and its distance from the shoulder, ambient temperature and duty hours, plus site constraints such as washdown, dust or corrosive atmosphere. A photograph of the existing installation with a legible nameplate closes most remaining gaps.

Representative inline drives include the R series inline helical unit in the 37 frame for moderate duty and the large R frame for heavy continuous service. Compare foot patterns and shaft options before fixing a new machine's interface - the frame decision is far cheaper on paper than after the baseplate is drilled.

FAQ

What is the difference between an R series and an RX series gearbox?
Both are coaxial helical units but are built on different internal arrangements with their own frame ranges and torque bands, so they overlap in duty rather than substituting for one another. Selection should follow torque, speed and mounting data rather than the letter.

Can an R series unit run in a vertical position?
Some builds can, with the correct oil level and breather position for that posture, but the seal and bearing duty changes and must be stated in the enquiry. For a downward process axis a dedicated vertical build is usually the better answer.

How do I know whether I need two or three stages?
Stage count follows from the ratio needed at the available motor speed. Deep reductions require a third stage; moderate ones do not. Ask for the delivered output speed at the stated input speed rather than the nominal ratio, then check the result against the machine's tolerance.

Does a higher frame always mean more safety margin?
Not automatically. A larger frame brings more thermal capacity and a bigger bearing set, but the margin that matters is the one measured against the actual duty cycle and overhung load. Oversizing without checking those two numbers simply moves the uncertainty.

How often should the oil be changed?
Follow the interval in the unit's manual, which depends on oil type, ambient temperature and running hours. The first change after running-in is the most important, because it removes the wear particles generated while the new gear surfaces bed in.