Helical Gearbox Selection: H/B Series Stages, Shafts, Cooling

A helical gearbox large enough that no geared motor covers it is, in our range, an H/B series industrial gear unit: the H/B series is built in one to four gear stages across 22 frame sizes, for motor powers of 2.52 to 5,366 kW, output speeds of 1.7 to 1,200 r/min and output torque up to 470,000 N·m. It comes in two types — YFH with parallel shafts and YFB at a right angle — and it is a modular unit-structure design, which means the stage count, the output shaft, the mounting and the cooling are four separate choices rather than one catalogue number. This guide takes them in the order the selection pages ask for them. Which family of gear unit you need in the first place is a different question, answered in our gear reducer selection guide.

Key definitions

• Helical gearing: teeth cut at an angle to the shaft axis, so each tooth pair comes into mesh progressively. More tooth contact, less noise, and an axial thrust the bearings have to absorb.
• Hardened tooth flank: a gear made from high-strength alloy steel, carburized and quenched so the working surface is hard while the core stays tough. It is what lets a given frame carry a much higher load.
• Stage: one gear mesh. An H/B unit has one, two, three or four, and the stage count sets the available ratio band.
• Modular unit structure: stages, shafts, mounting arrangements and cooling options are combined from a common set of housings and parts rather than designed per order.
• Assembly design (A, B, C, D): the arrangement of the shafts and the housing relative to the drive — a field in the type designation, fixed at order.
• Oil-supply factor f9: a factor in the thermal calculation that accounts for how the oil reaches the gears; 1.0 for horizontal units, 1.05 with forced lubrication, per Table 24.
• Thermal capacity: the power the unit can shed as heat continuously. A second, independent limit alongside mechanical strength.

Where the H/B series sits in our range

Series	Transmission	Sizes / frames	Motor or input power	Output speed	Max output torque
R	coaxial helical	13 (R17–R167)	0.12–160 kW	—	17,700 N·m
F	parallel-shaft helical	10 (37–157)	0.12–200 kW	—	16,800 N·m
K	right-angle helical-bevel	12 (K37–K187)	0.18–200 kW	—	49,400 N·m
H/B	modular helical (YFH) or bevel-helical (YFB), 1–4 stages	22 (1–22)	2.52–5,366 kW	1.7–1,200 r/min	470,000 N·m
ZDY / DBY	cylindrical and bevel-cylindrical, hardened teeth, national standards	ZDY 80–560, ZLY 112–710	nominal input to approx. 6,666 kW in the ZDY table	—	—
P	2- or 3-stage planetary	9–36	0.4–1,293 kW	0.19–60 r/min	2,600,000 N·m

Two boundaries are worth reading off that table.

Below, the geared-motor series. The F series tops out at 16,800 N·m and the K series at 49,400 N·m, and both arrive as a flanged geared motor with one nameplate if you want it. The H/B series is specified by the motor power that drives it, so the motor, its mounting and its alignment are part of your scope. That trade-off — one assembly against a gear unit plus a motor — is set out in our geared motor guide, and it is a real cost difference, not a formality.

Beside it, two alternatives at similar torque. If the output speed is below 60 r/min, the comparison is not another helical unit at all; it is the planetary series, and our planetary speed reducer guide explains why a single helical train runs out of ratio before it runs out of housing. If your specification calls for a gear unit built to Chinese national standards, that is the ZDY / DBY family: optimised centre distances and nominal ratios, interchangeable main parts, cylindrical ratios of 1.25–6.3 single-stage, 6.3–20 two-stage and 22.4–100 three-stage, and bevel-cylindrical ratios of 8–14 and 16–50. Its designation encodes a centre height (YFD B Y K 160 14 III S) where H/B encodes a frame number, which tells you the two families are specified from different starting points. All nine series sit side by side in our industrial gear reducer round-up.

Reading the type designation

The printed pattern is YFB 2 S H -10 -12.5 -A -CW, and each field is a decision made before the order:

Field	In the example	What it fixes
Series type	YFB	right-angle bevel-helical; YFH for parallel-shaft helical
Number of stages	2	one to four; sets the ratio band
Output shaft design	S	solid shaft; H hollow shaft, D hollow shaft with shrink disk
Mounting position	H	horizontal with feet; M horizontal without feet, V vertical
Size	10	one of frame sizes 1 to 22
Nominal ratio	12.5	the catalogue ratio, which is not always the calculated one
Assembly design	A	the shaft and housing arrangement; B, C, D are the alternatives
Input shaft rotation	CW	clockwise; CCW counter-clockwise

Choosing the stage count

The stage count is the field most often guessed, and it is the one the ratio decides outright. The nominal ratios printed in the thermal tables are:

Type	Stages	Nominal ratio band
H1SH	1	1.25–5.6
H2 / B2	2	5–28
H3 / B3	3	12.5–112
H4 / B4	4	80–450

Calculate the required ratio from your input speed — the tables are built around 750, 1,000 and 1,500 r/min — and the output speed the process needs, then read off the band. Note the deliberate overlaps: 12.5 to 28 is available in two or three stages, and 80 to 112 in three or four. Where you land in an overlap, do not pick the smaller number of stages by reflex. Fewer stages in a bigger frame and more stages in a smaller one can both meet the torque, and the deciding figures are the thermal check and the actual nominal ratio on offer.

The output shaft and the mounting

The output shaft is the choice that changes the installation rather than the gearbox. A solid shaft (S) is the conventional answer: a coupling, a base frame and an alignment job. A hollow shaft (H), or a hollow shaft with shrink disk (D), mounts the unit onto the driven shaft and removes all three — at the cost of a torque arm to hold the casing and a load path through the driven shaft that someone has to check.

Mounting is a separate field: H horizontal with feet, M horizontal without feet, V vertical. It is not only geometry. Horizontal units use dip lubrication as standard; forced lubrication is available on request and is used for vertical units. A unit ordered as V is therefore a different scope of supply from the same unit ordered as H, and the f9 oil-supply factor moves from 1.0 to 1.05 with it. Mounting position also fixes where the breather, the oil level indication and the drain sit, which is a maintenance question covered in our gear reducer maintenance guide.

The two checks this series makes mandatory

The selection pages state the principle plainly: a reducer's capacity is limited by mechanical strength and by thermal balance, and both have to be checked. The sequence is: calculate the transmission ratio; determine the nominal power using the driven-machine, prime-mover, starting and reducer safety factors; check the maximum torque; check the permissible additional radial force on the output shaft; confirm the actual ratio; then verify the thermal-balance power.

Thermal balance against a named cooling condition. The catalogue publishes thermal-capacity tables four times over — no auxiliary cooling, with a cooling fan, with a cooling coil, and with fan plus coil. Pick the condition you will actually install, not the most favourable one. If the absorbed power exceeds the figure for that condition, add cooling hardware; a larger frame is the expensive way to fix a heat problem. The same discipline applies on the ZDY family, where PG1 (no auxiliary cooling) runs 13–1,020 kW and PG2 (coil or oil-pump circulation) 43–1,780 kW depending on size and ambient air velocity — roughly a doubling of rating from cooling alone.

Peak torque and additional radial force. The continuous rating is not the duty. A drive that starts against a loaded machine, reverses or takes shocks has a peak the frame must survive, and anything overhung on the output shaft has to stay inside the permissible additional radial force for that size. Both are table look-ups against your duty, and both are ours to run.

The catalogue's own worked example is a blast-furnace feed conveyor drive — continuous, dusty, heavy — which is exactly the case where the cooling condition and f9 decide the outcome. For belt conveyors specifically, start from our conveyor gearbox selection guide, which sizes the drive from belt speed and drum diameter before it reaches a series name.

Sizing an H/B unit, step by step

1. State the duty: absorbed power at the driven machine, input speed, required output speed or ratio, output torque if you have it.
2. Take the ratio band and with it the stage count and the type (YFH parallel or YFB right angle) from the layout.
3. Determine the nominal power by applying the driven-machine, prime-mover, starting and safety factors, and compare it with the rating for that size and ratio.
4. Check maximum torque against starts per hour, load character and reversals.
5. Check permissible additional radial force if anything hangs on the output shaft.
6. Confirm the actual nominal ratio from the table and the output speed it gives you.
7. Verify thermal balance against the cooling condition you will install, and add a fan, a coil or both if needed.
8. Fix the scope: output shaft S, H or D; mounting H, M or V; assembly design A to D; input rotation CW or CCW.

Our recommendation

Specify the H/B series when the drive is past the geared-motor range but the output speed is still above 60 r/min — that is the band where a modular multi-stage helical or bevel-helical unit is the right machine, and where the alternatives are a planetary unit (too slow) or a national-standard ZDY/DBY unit (a specification requirement rather than an engineering one). Choose YFH or YFB from the layout drawing, the stage count from the ratio, and the output shaft from whether the unit can sit on the driven shaft. Then treat the cooling condition as a selection input: it is the one figure that changes the rating without changing a single mechanical number, and it is the one most often missing from an enquiry.

Send the absorbed power, input and output speeds, load character, starts per hour, any overhung load, the mounting arrangement and where the unit will stand through the enquiry form. Our Reducer Sales department and engineers return a frame size, a stage count, a complete type designation, the nominal power and permissible torque against it, the thermal rating for your cooling condition and a dimension drawing; the H/B series catalogue and every other series catalogue are on the gear reducers pages. Yanfeng Group Co., Ltd. has designed and built gear reducers and plastic weaving machinery in Pingyang, Wenzhou since 1987. Office hours are Monday to Saturday 08:00–17:30 and Sunday 09:00–17:30, China Standard Time (UTC+8).