Worm Gear Reducer Selection: S Series vs K Series Right-Angle

A worm gear reducer is a gear unit whose final stage is a worm and worm wheel instead of a gear pair, which makes it a right-angle drive by construction and lets one stage carry a ratio that a single gear stage cannot. In our catalogue two quite different machines answer that description, and buyers regularly ask for one and get quoted the other: the S series helical gear/worm reducer, which turns a shaft, and the SWL series worm-bolt lifter, which moves a load up and down a screw. This guide separates the two, then answers the question that actually decides a right-angle purchase — S series or K series — and runs the S-series sizing in the order our own selection tables do. Every figure comes from the S series catalogue and the series pages under gear reducers.

Key definitions

• Worm and worm wheel: a screw-form worm meshing with a toothed wheel on an axis at 90 degrees to it. The pair is the output stage of the S series.
• Helical-worm unit: a two-stage arrangement, helical gears first and the worm set second, which is what the S series is. The helical stage is why the ratio range starts at about 6.8 rather than lower.
• Ratio i: input speed divided by output speed. A 1,450 r/min motor through i = 29 gives 50 r/min at the output shaft.
• Permissible output torque T2: the torque a frame size is rated for continuously at a given ratio — the figure in the selection table, not a peak.
• Service factors f1, f2, f3: multipliers applied to the required torque for the driven machine and its load character, the prime mover and the number of starts per hour.
• Mounting position M1 to M6: the orientation the unit is installed in. It sets the oil quantity and the positions of the breather, oil sight glass and drain plug.
• Self-locking: a drive that holds its load when the input stops, without a brake. It is a property the catalogue either claims or does not; see below.

Two machines, one search term

	S series helical gear/worm reducer	SWL series worm-bolt lifter
What it does	reduces speed and turns an output shaft at 90 degrees	raises, lowers, pushes or positions a load on a trapezoidal screw
Output motion	rotary	linear
Sizes	7 (S37 to S97)	11 (YFSWL1 to YFSWL100)
Rating	93 to 3,990 N·m output torque	10 to 1,000 kN lifting force
Screw / wheel	worm and worm wheel on the output stage	worm and worm wheel driving a Tr22x5 to Tr150x24 screw
Self-locking	not claimed in the catalogue	yes — holds the load when stopped
Lubrication	oil, with breather, sight glass and drain plug set per mounting position	calcium-based grease ZGN-1 or ZGN-2, -20 to 100 C
Duty	continuous, per the service factors	maximum 20% ED calculated over a 30-minute cycle

If the machine needs a turning shaft, the S series is the unit. If it needs something to travel in a straight line and stay where it is put, the SWL jack is the unit, and the screw jack selection guide is the place to start — including the linkage factor that derates each jack when several are driven together.

S series or K series: the right-angle decision

Both are right-angle reducers and both are sold as bare gear units or as geared motors with a flanged AC motor. They are not interchangeable.

	S series helical-worm	K series helical-bevel
Output stage	worm and worm wheel	spiral bevel gear set
Frame sizes	S37 to S97 (7)	K37 to K187 (12)
Input power	0.12 to 22 kW	0.18 to 200 kW
Ratio range	approx. 6.8 to 290	approx. 5.36 to 197
Permissible output torque	93 to 3,990 N·m	156 to 49,400 N·m
Load direction	forward and reverse	one direction only; state the rotation when ordering
What the catalogue emphasises	compact, quiet, a wide ratio range in a single unit	high torque and high efficiency at high power

The decision reduces to two questions in this order. How much torque and power? Above 22 kW or 3,990 N·m the S series does not have a frame and the K series is the only answer. How much ratio? Below about 197 both can reach it; above that only the S series can without stacking two units. In the band where both fit — modest power, high ratio, a tight installation — the S series is the smaller and quieter unit, and the fact that it takes load in either direction removes the rotation question the K series forces you to answer at order stage. For the full four-family picture, including the in-line R and parallel-shaft F series, see the gear reducer selection guide.

One further case belongs here. If the drive has to change direction but not speed, neither of these is right: that is a bevel gearbox, the T series, at a ratio as low as 1:1 and with no motor of its own.

The seven S series frames

Frame size	Input power	Ratio range	Permissible output torque
S37	0.12–1.1 kW	6.8–157.43	93 N·m
S47	0.12–1.5 kW	7.28–201	170 N·m
S57	0.18–3 kW	7.56–217.41	245 N·m
S67	0.25–5.5 kW	8.06–256.47	520 N·m
S77	0.75–7.5 kW	up to approx. 288	1,170 N·m
S87	0.75–15 kW	up to approx. 288	2,130 N·m
S97	1.5–22 kW	up to approx. 288	3,990 N·m

Two things are worth reading off this table before you shortlist. The torque roughly doubles from one frame to the next across most of the range, so a required torque that lands just above a frame's rating pushes you a whole size up rather than a little way — which is the usual reason a service factor applied carelessly costs money. And the ratio ceiling rises only modestly from S37 to S97 while the torque rises more than fortyfold: on this series you buy torque with frame size, not ratio.

Sizing an S series unit in six steps

1. Fix the output speed n2 from the machine, not from the motor: the drum speed, the screw feed rate, the stirrer speed you actually need. The ratio follows: i = motor speed / n2, taking 1,450 r/min for a 4-pole motor at 50 Hz.
2. Find the torque the machine needs, T1. From the load and the radius it acts on, or from the absorbed power: T1 (N·m) = 9,550 x P (kW) / n2 (r/min).
3. Apply the service factors. The requirement is T_N >= T2 x f1 x f2 x f3, where f1 comes from the driven-machine class — uniform load, moderate shock, heavy shock — and the daily operating hours, f2 from the prime mover and f3 from the number of starts per hour. The catalogue rates the unit for steady load, a stated daily operating time and a limited number of starts; anything else goes through these factors.
4. Read the frame size from the table above and then from the selection table: the smallest frame whose permissible output torque at your ratio is at least the figure from step 3, and which offers the motor power you need. Both conditions have to hold; a frame that has the torque but not the motor rating is not a selection.
5. Choose the mounting structure. Foot-mounted (YFS), B5 flange (YFSF), hollow output shaft (YFSA), hollow shaft with B5 flange (YFSAF), hollow shaft with B14 flange (YFSAZ), foot plus hollow shaft (YFSA...B) and foot plus flange (YFS...F) are all listed in the type-designation key. A hollow shaft onto a driven shaft removes a coupling and an alignment job.
6. Fix the mounting position and shaft side. M1 to M6, output shaft on side A, B or A+B, terminal box at 0, 90, 180 or 270 degrees. State all of it on the order: the oil quantity and the positions of the breather, oil sight glass and drain plug are set for that position at the factory, which is why a unit remounted in a different position on site is no longer lubricated the way it was built.

The result is a single type designation — enterprise code, series letter, mounting code, frame size, motor power and poles, ratio or output speed, mounting position, output-shaft side and terminal-box position. The printed key uses a K-series unit as its worked example, YF K A 67 B Y-4 -4p i9.66 M1 B 180 2; an S-series order reads the same way with S in place of K. A supplier who cannot return that string with a permissible torque against it has not done the selection.

What our catalogue does not claim

Worm gearing carries two reputations that our data supports for one product and not the other, so be precise about which machine you are buying.

• Self-locking. The SWL jack catalogue states plainly that the unit is self-locking when stopped, so the load is held without a brake. The S series catalogue makes no self-locking claim. If a conveyor incline, a hoist or a gate has to stay put when power is lost, specify a brake motor or a mechanical holding device and do not rely on the worm stage to do it.
• Efficiency. Our data gives efficiency figures only for the SWL jack — 13 to 23% on the normal (P) worm ratio and 8 to 13% on the slow (M) ratio, which is what a lifting screw costs you. No efficiency figure is published for the S series reducer, so do not transfer the jack's numbers to it, in either direction. Ask us for the figure for the frame and ratio you are considering and we will confirm it from the master tables.

The construction is common to the range: a cast housing with bolted cover and gasket, breather cap, oil sight glass and drain plug, the worm and worm wheel with a helical gear and pinion pair on the input side, an output shaft on rolling bearings with shaft sleeves, circlips and flat and lip-type oil seals. The gears are precision-ground alloy steel, the bearings high-grade ball bearings, and the finish the three-layer high-density polymer paint used across all nine series, which seals out dust and tolerates high temperature. The gear grinding, hobbing and inspection equipment behind that is listed on our production capacity page.

Our recommendation

Name the machine before the size. If the output is a rotating shaft at a right angle, you want an S or K series reducer and the torque figure decides which; if the output is linear travel that has to hold position, you want an SWL jack and the worm reducer is the wrong machine. Within the right-angle choice, take the S series when the duty sits inside 22 kW and 3,990 N·m and the ratio is high, and the K series when it does not. Never specify an S series unit on the assumption that the worm stage will hold a load.

Send the output speed, the torque or absorbed power, the load character, the daily running hours, the starts per hour and the space you have through the enquiry form. Our Reducer Sales department and engineers return a frame size, a mounting structure, a complete type designation, the permissible torque against it and a dimension drawing. Yanfeng Group Co., Ltd. has built 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).