Gear Motor Output Shaft and Flange: Specifying the Mechanical Interface

Two gear motors with identical electrical data can behave completely differently on the same machine, and the difference is almost always the mechanical interface: the output shaft, the flange, and the tolerances nobody put on the drawing. A coupling that slips 3° on every reversal, a flange that pulls the shaft out of alignment by 0.2 mm, a shaft seal that weeps after 200 hours — none of these appear in a torque table.

This guide covers how to write the interface specification, in the order it should be decided: shaft end, then flange register, then load limits, then sealing, then the tolerance callouts that make the drawing enforceable. The examples use our own 28 mm and 58 mm platforms, so the numbers can be checked against published product data.

Checking an output shaft against a mounting plate on the bench: caliper on the shaft, pilot bore and bolt circle on the plate.

The Interface Is Four Decisions, Not One

Most RFQs specify "6 mm shaft, 4-hole flange" and consider the interface closed. In practice four separate decisions are being made, and each one constrains the next:

1. How torque leaves the motor — round shaft with a clamp coupling, D-cut with a set screw, keyway, spline, thread, or a gear cut directly on the shaft.
2. How the motor is held — flange register (pilot diameter), bolt circle, bolt size, and the direction the mounting face looks.
3. What the shaft is allowed to carry — overhung load at a stated distance, axial load, and whether the load reverses.
4. What has to stay inside — the seal at the shaft exit, which is where most IP ratings are lost.

Change one and the others move. A keyway makes the shaft weaker in torsion but allows three to five times the torque of a set screw on a flat. A larger pilot diameter improves concentricity but adds 4–6 mm to the housing. Deciding them in this order avoids the usual loop of re-quoting. The shaft questions below apply to every planetary gear motor we build, and to the worm gear range with one extra variable: the output direction.

Shaft End Options and What Each One Costs

For a 6 mm shaft the D-flat is normally cut 1.0 mm deep, which is 15–17 % of the diameter; going deeper than 20 % starts to bend the shaft under load and weakens the bearing seat. For a 4 mm shaft, 0.5 mm is the practical flat. Keyways follow the shaft diameter and the dimensional series defined for parallel keys and keyways: 2 × 2 mm for 6 mm shafts, 3 × 3 mm for 8 mm and 10 mm shafts, which is the DIN 6885 series most Asian and European drawings still reference.

What a set screw actually holds

An M3 set screw on a 6 mm D-flat, tightened to 1.2 N·m, gives a slip torque of roughly 0.3–0.6 N·m depending on the hub material and how much of the flat the screw lands on. A 2 × 2 mm key, 8 mm long, in a mild steel hub shears at about 2.9 N·m. That is the whole argument for keyways in one comparison: five times the torque capacity for one extra machining operation.

If your gearbox output is above 1.5 N·m and the load reverses, a D-cut with a set screw is the wrong answer regardless of what the prototype did for a week.

Flange and Mounting: The Pilot Diameter Does the Work

Bolts locate nothing. They clamp. The feature that decides whether the output shaft stays concentric with the driven part is the register — the machined step on the gearbox face that enters a matching bore in your mounting plate.

On our 28 mm platform the gearbox is 27.5 mm long at one stage and 52.1 mm at four stages, so the mounting face position changes with the ratio. If the plate is designed around a one-stage unit and the order later moves to a four-stage unit for more torque, the shaft end moves 24 mm. Fix the ratio before you fix the plate.

DM-28PM395 in the 28 mm metal planetary housing: the same flange pattern serves ratios from 3.7:1 to 369:1, but the shaft sits further out as stages are added.

Right-angle boxes such as the DM-58WM555 worm gear motor add a second question: whether the output is a single shaft, a double shaft, or a hollow bore. The 40.1 mm gearbox with 2–122 rpm output and 1–80 kgf·cm range is often chosen for exactly this reason — the right-angle layout lets the motor sit parallel to the machine frame instead of sticking out of it.

DM-58WM555 with the 40.1 mm right-angle worm gearbox: the output shaft direction is a specification decision, not an afterthought.

Overhung Load: The Number That Is Missing From Most RFQs

Put a pulley on the shaft and the bearing now carries a side load. The shaft is a cantilever, and the bending stress at the bearing seat is:

σ = 32 × F × L / (π × d³)

where F is the overhung load in newtons, L the distance from the bearing to the load centre in metres, and d the shaft diameter in metres. With an allowable bending stress of 120 MPa for a carbon steel shaft — roughly half the yield of a 45# steel — the permissible load at 10–15 mm from the bearing works out as:

The last column is the one that usually governs. The shaft almost never breaks; the bearing brinnells first, and the symptom is a rumble that appears after a few hundred hours and gets blamed on the gears. Deflection is not the limiter either: with a modulus of about 200 GPa for steel, a 6 mm shaft under 50 N at 10 mm bends about 0.0013 mm — invisible next to the bearing clearance. Use the lower of the two columns, and halve it again for belt drives, where the belt tension adds a radial pull that the torque calculation never sees.

Note that a belt or chain drive multiplies the load. A 40 mm pulley with 2 N·m of torque needs 100 N of tangential force, and a belt tensioned to 1.5× that adds 150 N of radial load — right at the limit of an 8 mm shaft at 15 mm. That is why conveyor and belt-driven applications should state the pulley diameter on the RFQ, not just the torque.

Sealing Happens at the Shaft, Not the Flange

A gearbox can be perfectly sealed at the housing joint and still fail an IP65 test, because the shaft exit is the only place with a moving surface. Three details decide it:

• Lip seal or O-ring on the shaft. A lip seal running on a ground surface at Ra 0.8–1.6 µm holds; the same seal on a turned surface at Ra 3.2 µm wears through in a few hundred hours.
• Grease selection. A high-viscosity grease stays put but channels at −20 °C; a low-viscosity grease seals less and migrates onto the seal lip.
• Shaft surface hardness. Below about 40 HRC the seal groove polishes a step into the shaft and the leak starts there.

IEC 60529 defines what the IP number actually tests — IP54 is dust-protected plus splash, IP65 is dust-tight plus water jet, IP67 is temporary immersion. If a tender asks for IP65 on a shaft that exits horizontally, the seal has to survive a jet at the specified pressure and distance, which is a different requirement from a splash.

Tolerances Worth Putting on the Drawing

Two of these get skipped most often and cause the most field trouble: flat angular position (a hub that only fits with the screw at 90° to where the drawing says) and perpendicularity of shaft to flange (a motor that mounts flat but runs with a wobble that eats the coupling in weeks).

For scale, the DM-28PM395 planetary gear motor is a 28 mm platform with four ratio stages from 3.7:1 to 369:1 and a gearbox length from 27.5 mm to 52.1 mm — which means the same flange drawing serves four different shaft positions. That is normal, and it is exactly why the drawing has to call out the ratio, not just the model.

Five Interface Mistakes That Reach Production

1. Specifying a shaft diameter without a tolerance. "6 mm shaft" means anything from 5.95 to 6.05 depending on the shop. A clamp coupling designed for 6.00 h7 will not clamp a 5.95 shaft properly, and the joint slips under reversal.

2. Using the flange bolts to pull the motor into alignment. If the register is wrong, tightening the bolts bends the housing. The gear mesh then runs with a 0.1 mm offset, noise rises 3–5 dB(A), and the first gear stage wears out early.

3. Forgetting the cable exit direction. A connector at the top of a vertically mounted gearbox collects condensate. Rotate the end cap or specify the exit at 6 o'clock.

4. Loading the shaft with a press fit. Pressing a hub onto a 6 mm shaft puts an axial load into the bearing that it was never rated for. If the joint needs a press fit, specify a bearing arrangement that takes thrust, or use a clamp instead.

5. Leaving the shaft finish to the supplier. A turned finish at Ra 3.2 µm is fine for a clamp coupling and fatal for a lip seal. Put the finish on the drawing next to the seal, not in a general note.

Writing the RFQ: Twelve Lines

A drawing plus these twelve lines gives a supplier everything needed to quote without a follow-up call:

1. Gearbox model and ratio, or the output speed and torque required
2. Shaft diameter, tolerance class, and length from the flange face
3. Shaft end type, with flat depth or keyway dimensions and angular position
4. Shaft runout and end play limits
5. Flange type, pilot diameter, bolt circle, hole count and hole size
6. Mounting face orientation relative to the cable exit
7. Maximum overhung load and the distance from the bearing
8. Axial load, if any, and whether it reverses
9. Seal requirement and the IP level it must meet
10. Shaft material and surface finish at the seal
11. Connector type, lead length and colour code
12. Annual volume and the sample quantity needed for validation

Lines 7 and 8 are the ones that change the internal bearing choice, and therefore the price. A quotation without them is a guess.

If the interface is not standard, the drawing usually starts from an exploded view of the existing platform rather than a blank sheet — the housing, bearing seats and seal grooves are already defined, and only the shaft and flange change. For heavily modified interfaces, our custom motor design process covers the drawing approval and first-article steps.

FAQ

Can I get a D-cut on a shaft that is already keyed?

Not on the same shaft section. The keyway removes material from one side and the flat from another; together they can reduce the section below the minimum for the torque. Specify one or the other, or move the flat to a different axial position.

What shaft tolerance should I ask for with a clamp coupling?

h7 on the shaft and H7 on the coupling bore is the usual pairing for a clamp collar that has to transmit torque without slipping. If the coupling is a printed or cast part, h9 on the shaft is more forgiving and still locates well enough.

How much overhung load can a 6 mm shaft take?

At 10 mm from the bearing, the shaft itself can carry about 255 N before reaching 120 MPa bending stress, but the bearing is the real limit and usually allows 40–70 N. Specify the load at the actual distance and let the supplier confirm the bearing, because bearing size varies between gearbox stages.

Does a longer shaft reduce or increase the load limit?

It reduces it, and quickly. Bending stress scales linearly with the distance from the bearing, so doubling the shaft length from 10 mm to 20 mm halves the permissible load. A long shaft for a pulley is the most common cause of premature bearing noise in small gear motors.

How do I check concentricity after assembly?

Mount the gearbox on its register, put a dial indicator on the shaft 10 mm from the flange, and rotate the output by hand one full turn. Total indicated reading above 0.05 mm means either the register fit is too loose or the plate bore is off-centre. Our quality control process runs this check on every unit, and it takes about twenty seconds per motor with a fixture.