What Parameters Do You Need to Submit to Select an SWL Screw Jack?

Selecting an SWL screw jack is not a matter of submitting "a model number". You are submitting six groups of data:

1. Load — dynamic load, static load and whether shock is present, each stated separately;
2. Stroke — effective stroke plus allowance;
3. Speed — lifting speed, or screw speed plus an acceptable lead;
4. Duty cycle and operating pattern;
5. Mounting and guiding;
6. Drive and safety requirements — self-locking, braking, end limits, anti-fall.

Stroke has to be settled at enquiry stage. It is not something you add once the model is chosen: the catalogue's own dimension rows are laid out as "model + stroke + offset", which makes stroke a structural variable. A model number without a stroke is not an enquiry.

Why an SWL Carries More Parameters Than an Ordinary Gearbox

An ordinary gearbox is a pure transmission component. Speed and power go in, speed and torque come out, and its parameters stay inside those two pairs.

A screw jack is an actuator. It turns rotation into linear lifting. So on top of the transmission group there is a whole second group — geometry and motion: how far the load travels, how fast, how it is guided, and where the unit is mounted.

That is why an enquiry that stops at "I need an SWL and here is the load" rarely earns a direct answer. What is missing is not model information — it is motion information.

The Six Data Groups at a Glance

The sections below take each group in turn.

Load: Static, Dynamic and Shock

Static load is the load at rest or at constant speed. Dynamic load is the load during start-up, acceleration and stopping. The two can differ widely, and a lifting mechanism is at its most demanding at the moment it starts or stops.

Three things need to be stated when you submit:

1. what the static load is;
2. roughly what the dynamic load peak is, or whether starts and stops are gentle;
3. whether shock is present — drop loading, collision, jamming.

If you can only give one figure, say whether it is the static or the dynamic one. Defaulting to static is the most common source of underestimation.

Stroke: Why It Has to Be Settled First

This is where an SWL differs most visibly from an ordinary gearbox.

Open a catalogue dimension table and the rows are laid out as "model + stroke + offset", with the dimension columns following behind. In other words, change the stroke and the structure changes with it. Stroke is not an accessory specified after the model — it is an input that determines the structure.

So the correct order is:

1. fix the effective stroke — how far the load actually has to travel;
2. add the allowance — what installation, adjustment and cushioning require;
3. hand both figures to the supplier together.

A model number without a stroke is not an enquiry. This is the most common extra round-trip in SWL sourcing.

Speed and Duty Cycle: Two Quantities You Can Work Out Yourself

Lifting speed follows from screw speed and lead:

v = n_s · P

n_s is screw speed (rpm), P is screw lead (mm per revolution), and v is lifting speed (mm/min).

Screw speed itself follows from motor speed and ratio:

n_s = n_motor / i

n_motor is motor speed and i is the ratio.

Put the two together and the constraint is obvious: of speed, lead and screw speed, only two can be fixed independently. When you submit, give either the target lifting speed, or the target screw speed together with an acceptable lead — one or the other, not all three written down as fixed.

Duty cycle measures the loaded share of the running time:

D = t_on / (t_on + t_off)

t_on is loaded running time and t_off is the interval. The expression is simple, yet on site it is often replaced by a phrase.

"Occasionally" is not a parameter. Write three sentences instead: how long each lift lasts, how long it stops between lifts, and how many lifts per day. Those three sentences carry far more usable information than a one-word description ever does.

Mounting and Guiding: A Guide Is Not Optional

Whether a guide is needed depends on the side load.

• If the load stays vertical throughout the lift, the screw mainly carries axial force;
• as soon as a side load appears — off-centre load, wind load, reaction from a guide rail, horizontal thrust from the lifted part — and there is no guide, that side load turns into additional bending moment in the screw.

The consequence is not a slight loss of accuracy. It is a bent screw, uneven nut wear and seal failure. Where there is side load there must be a guide, and that is not an option.

Four things need to be stated:

1. the mounting orientation (upright, inverted, horizontal pushing, and so on);
2. whether side load exists, and roughly where it comes from;
3. whether rails or a guiding structure already exist, and who supplies them;
4. how far the centre of gravity of the lifted part sits off the screw axis.

Drive and Safety: Self-Locking Is Not Braking

State the drive type: manual, direct motor, gearmotor, or servo.

On the safety side, four items must each be answered:

These four items belong to whole-machine safety design. What the supplier provides is the lifting actuator; the supplier does not replace whole-machine safety verification. Whether end limits, anti-fall and overload protection actually hold depends on the machine designer's own design and validation.

Condition Table: What Each Situation Implies

Pre-Enquiry Checklist

Before enquiring:

1. Confirm static and dynamic load, and whether starts and stops are gentle.
2. Confirm the effective stroke, and add the installation and cushioning allowance.
3. Confirm the target lifting speed, or determine screw speed and an acceptable lead.
4. Confirm the duration of each lift, the interval, and the number of lifts per day.

Structure confirmation:

1. Confirm the mounting orientation and the available space.
2. Confirm whether a side load exists, and where it comes from.
3. Confirm who supplies the guiding (machine side or jack side).
4. Confirm the offset of the lifted part's centre of gravity from the screw axis.

Safety confirmation:

1. Confirm whether the position must be held after power loss.
2. Confirm how the upper and lower limits are detected and stopped.
3. Confirm whether a person may be beneath the lifted part; if so, the design must follow functional safety requirements.

Common Errors and Scope

Common errors:

1. Reporting a single "load" figure, with no split between dynamic and static and no mention of shock.
2. Enquiring before the stroke is fixed, treating stroke as an accessory that follows the model.
3. Writing speed, lead and screw speed all down as fixed, so the three contradict each other.
4. Using "used occasionally" in place of an operating pattern.
5. Not mentioning a side load, and not adding a guide.
6. Treating self-locking as braking, and assuming the load cannot descend after power loss.
7. Expecting the supplier to carry out whole-machine safety verification on your behalf.

Scope of this article:

• The six data groups, the three relations and the enquiry template here apply to preparing selection data for the SWL series of worm gear screw jacks.
• This article gives no numeric values for load, stroke, speed, lead or screw thread specification, and no rated load for any individual model. Those have to be taken from the product page for the specific model, or confirmed by engineering.
• This article does not replace whole-machine safety verification. End limits, anti-fall protection and overload protection are the design and validation responsibility of the machine designer.
• This article does not replace the installation manual for a specific model.

Enquiry Template (Copy and Paste)

1. Application and equipment type:
2. Static load:              kg      Dynamic load peak:            kg
3. Shock present (drop / impact / jamming):
4. Effective stroke:         mm      Allowance:                    mm
5. Target lifting speed:     mm/min  (or target screw speed ___ rpm, acceptable lead ___ mm/rev)
6. Duration per lift:        s       Interval between lifts:       s      Lifts per day:
7. Mounting orientation:             Space restriction:
8. Side load: yes / no               Source:
9. Guiding: supplied by machine side / required on jack side
10. Drive type: manual / motor / gearmotor / servo
11. Must position be held after power loss: yes / no
12. May a person be beneath the lifted part: yes / no

The value of this template is in its last line: if item 12 is answered "yes", the conversation should move straight to a safety solution, not to comparing models.

About This Article and the Publishing Entity

This article was written by the technical team of Chengfa Reducer (Changzhou) Co., Ltd. (CHINFAR) and published after internal technical review. The SWL series of worm gear screw jacks is supplied by Chengfa Reducer (Changzhou) Co., Ltd.; CHINFAR products are built and delivered within the joint manufacturing system shared with HUIGUANG.

Related product and background reading:

• SWL series worm gear screw jacks
• What data a gearbox enquiry needs: the selection input list
• Contact the CHINFAR technical team

Next Step

Fill in the template and send it to sales@chinfar.com. If you only have part of the data, send it anyway — but mark which items are still open, so the reply points straight at the gap instead of starting with another round of questions.

To be explicit: neither this article nor the supplier replaces whole-machine safety verification. Where people, falling loads or suspended loads are involved, the machine designer must design and validate independently against the applicable functional safety requirements.