Screw Jack Selection Guide: Sizing an SWL Worm Gear Jack

A screw jack is sized from four numbers you already know about the machine it goes into: the load in kilonewtons, the travel in millimetres, how fast the load has to move, and what share of each half-hour the jack actually runs. Those four pick the frame size, the worm ratio and the structural type; everything else in the model code follows from the way the jack is mounted. Yanfeng Group Co., Ltd. builds the SWL series worm gear screw jack in Pingyang, Wenzhou — eleven sizes, YFSWL1 to YFSWL100, covering 10 kN to 1000 kN of lifting force — and this guide runs the selection in the order our catalogue does. Every figure below comes from the SWL series page and the SWL catalogue PDF.

Key definitions

• Screw jack (worm-bolt lifter): a worm and worm wheel driving a trapezoidal (Tr) screw, so each turn of the input shaft moves the load a fixed distance.
• Maximum lifting force: the compressive load a size is rated for. On the SWL series the maximum tension load is the same figure for each size.
• Travel: the usable stroke of the screw in millimetres. It is specified in the model code, not chosen afterwards.
• Self-locking: the worm pair holds the load when the input stops, so position is maintained without a brake.
• Structural type 1 / type 2: in type 1 the screw travels axially and does not rotate; in type 2 the screw rotates and a travelling nut moves along it.
• Duty cycle (% ED): the running share of a 30-minute cycle. The SWL series is rated for a maximum of 20% ED.
• Linkage factor: the derating applied when one drive turns several jacks through a common line shaft.

The eleven SWL sizes

Size	Max lifting force	Max tension load	Screw thread
YFSWL1	10 kN (1 t)	10 kN	Tr22x5
YFSWL2.5	25 kN (2.5 t)	25 kN	Tr30x8
YFSWL5	50 kN (5 t)	50 kN	Tr42x8
YFSWL10	100 kN (10 t)	100 kN	Tr58x12
YFSWL15	150 kN (15 t)	150 kN	Tr58x12
YFSWL20	200 kN (20 t)	200 kN	Tr65x12
YFSWL25	250 kN (25 t)	250 kN	Tr90x16
YFSWL35	350 kN (35 t)	350 kN	Tr110x16
YFSWL50	500 kN (50 t)	500 kN	Tr120x16
YFSWL75	750 kN (75 t)	750 kN	Tr130x18
YFSWL100	1000 kN (100 t)	1000 kN	Tr150x24

Across the range the screw diameter d3 runs 16.5–124 mm and the screw pitch 5–24 mm. Maximum travel is 300 mm on YFSWL1 and 2000 mm from YFSWL35 upwards, and can be increased when the load is in tension. Unit weight, excluding travel, runs 5.5–748 kg.

Note the size number is the rating in tonnes: YFSWL25 is a 25-tonne jack. That makes the first pass of selection quick, but it is the kilonewton figure, after service factors, that has to be checked.

P or M: choosing the worm ratio

Every size is offered with two worm ratios. P is the normal-speed ratio, M the slow-speed ratio, and the choice sets both how far the load moves per input turn and how much torque the drive has to supply.

	P (normal speed)	M (slow speed)
Worm wheel ratio	approx. 1/8 – 1/15	1/22 – 1/40
Travel per worm revolution	1.0 – 1.565 mm	0.227 – 0.55 mm
Worm torque at full load	6.2 – 2040.9 N·m	2.9 – 1278.3 N·m
Efficiency	13 – 23 %	8 – 13 %

On YFSWL2.5, for example, the two ratios are 1/8 (P) and 1/25 (M). No-load torque across the whole series is 0.02–39.2 N·m. The practical trade is straightforward: P reaches a given travel in fewer input revolutions, M asks less torque of the drive and positions more finely. Where a jack is being driven by a geared motor rather than by hand, the torque figure in this table is what sets the motor size, not the lifting force.

Sizing an SWL jack in seven steps

1. Start from the load per jack. Take the heaviest load one jack carries, not the total load on the platform, and multiply it by the service factor f1: 1.0–1.3 where there is no shock load and inertia is small, 1.3–1.5 for moderate shock, 1.5–3.0 for heavy shock and large inertia.
2. Allow for linkage. If several jacks are driven together, the catalogue applies a linkage factor that reduces the capacity you can count on from each jack. Give the number of jacks with the load; it changes the size you need.
3. Fix the travel. Specify the stroke in millimetres in the model code. Check it against the size's maximum (300 mm on YFSWL1, up to 2000 mm on YFSWL35 and larger), and say if the load is in tension, which allows longer travel.
4. Choose P or M. Work from the required lifting speed and the travel per worm revolution in the table above, then check the worm torque at full load against the drive you intend to use.
5. Choose the structural type and assembly form. Type 1 if the screw must travel without rotating, type 2 if a travelling nut should move along a rotating screw. The four basic assembly forms are A1 (screw travels upward), B1 (screw travels downward), A2 and B2 (rotating screw, nut moving up or down).
6. Choose the screw-end coupling and accessories. Four couplings are available — I flat head, II roller head, III fork head, IV keyhole head — plus Z for a protective tube and F for an anti-rotation device.
7. Check duty, speed and stability. Duty must stay within 20% ED over a 30-minute cycle and input speed within 1500 r/min, and the drive's starting torque should be at least 200% of the running torque. Required input power follows P = T × n / 9550, with T in N·m and n in r/min. Long screws also need the buckling (critical load) and critical rotation speed checks set out in the catalogue.

The result is a single model designation, in the form YFSWL 2.5 M 1 A 500 Z I — series, size, ratio, structural type, assembly form, travel in mm, accessory, screw-end coupling. A supplier who cannot return that string, with a permissible load against it, has not done the selection.

Driving several jacks from one line shaft

Lifting platforms rarely use one jack. The SWL selection pages give a linkage factor for jacks driven together from a single source:

Jacks driven together	Linkage factor
1	1.00
2	0.95
3	0.90
4	0.85
5–8	0.80

The usual way to distribute the drive is through T series spiral bevel gearboxes, which turn the line shaft through 90 degrees. Ten frame sizes, T2 to T25, with standard ratios of 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 4:1 and 5:1, cover the layouts a jack system needs, and the same box can also be used as a speed increaser depending on which shaft drives. One rule matters when boxes are chained in series: the box nearest the motor carries the sum of the torque passing to every box downstream of it, so it is sized for the whole system, not for one jack.

Duty cycle, environment and lubrication

A screw jack is an intermittent machine. Beyond the 20% ED limit, the constraints to design around are the operating environment and the lubricant. The SWL series is lubricated with calcium-based grease ZGN-1 or ZGN-2, rated −20 to 100 °C. The working environment should be indoors and free of rain ingress, with normal factory dust levels, an ambient temperature of −15 to 40 °C and relative humidity below 85%. In dusty locations or outdoors, specify the Z protective tube — or a bellows — to keep the screw clean; a worn screw thread is the usual reason a jack loses travel accuracy.

Our recommendation

Size the jack from the load on the worst-loaded unit, with the service factor applied, before you think about the model code. Then decide P or M from the lifting speed, and only then the mounting details. Two mistakes account for most undersized jacks we are asked to replace: selecting on nominal load without the f1 service factor, and selecting a group of jacks as if each carried an equal share of a platform load that is in fact off-centre.

For context on where the SWL series sits in our range, the top industrial gear reducer series round-up deliberately excludes it — the screw jack and the T series each do one job rather than acting as general-purpose reducers — and the supplier verification guide sets out the factory, machining and certification checks behind all nine series.

Send the load per jack, the travel, the required lifting speed, the number of jacks and the duty pattern through the enquiry form. Our Reducer Sales department and engineers return a size, a worm ratio, a complete model designation and a dimension drawing. The full range is listed under gear reducers; office hours are Monday to Saturday 08:00–17:30 and Sunday 09:00–17:30, China Standard Time (UTC+8).