Sizing a worm gear screw jack is less about brand names and more about force, length, and heat. Anand Gears catalogue machine screw jacks cover 5 kN to 350 kN in translating (TP), rotating (TPR), and keyed forms. This note walks through the inputs a maintenance or design engineer should collect before RFQ. Units throughout: mm and kN.
1. Load per jack (static vs dynamic)
Static load is the force the jack must hold at rest — platform mass, tool weight, process reaction. Dynamic load is higher during acceleration, impact, or when friction and misalignment add side force. On multi-jack systems, do not divide total load by four and stop: uneven load share means some corners take more. Design each jack for a credible worst-case share.
Selection guidance on our product material starts with a safety factor of about 1.5–2× on working load, then still check duty, buckling, and side load. Pick a catalogue class (AG-SJ-05 … AG-SJ-350) whose published capacity sits above that result.
2. Travel (stroke) in millimetres
Define usable stroke, not only overall screw length. Add clearance for bellows compression, limit-switch overtravel, and nut engagement at both ends. Long strokes need a free length check for buckling when the screw works in compression.
3. Speed and duty cycle
Lifting speed depends on screw pitch, worm ratio, and input rpm. Catalogue gear ratios include 1/4, 1/8, 1/16, 1/24, and 1/32 options depending on size — slower ratios reduce input torque demand and help self-locking behaviour on trapezoidal screws.
Duty cycle is critical: worm gear screw jacks are typically intermittent-duty positioning devices. If the jack cycles almost continuously, heat in the worm mesh and nut rises. High duty often points toward a ball screw jack (with a brake), oil-bath lubrication options, or a different actuator. State on/off time and cycles per hour honestly on the RFQ.
4. Self-locking
Trapezoidal (Tr) screws with appropriate pitch and worm ratios are commonly self-locking: the load holds without a brake when power is off. Ball screws are efficient and usually not self-locking — they need a motor brake or external hold. Confirm hold requirements before choosing screw type. See also trapezoidal vs ball screw jacks.
5. Buckling and slenderness (compression)
When a long screw pushes a load (column action), Euler buckling can govern before pure compressive yield. Critical free length depends on screw diameter, end conditions (fixed, guided, free), and material. For long strokes, treat buckling as a hard limit — shorten unsupported length, increase screw diameter/class, add guides, or switch to a configuration that keeps the screw better supported (e.g. rotating screw with fixed ends where layout allows).
6. Upright vs inverted
Upright base mounting: housing on the structure, screw extends upward — the most common platform and table layout. Inverted: screw points down — useful when the jack hangs from a frame or the load attaches below. Clevis, top-plate, and trunnion mounts appear when the load path pivots. Orientation changes grease path and bellows layout; state it early.
7. Translating (TP) vs rotating (TPR) vs keyed
- Translating screw (TP) — screw does not rotate; nut in the worm wheel drives linear motion. Preferred when the load must not spin.
- Rotating screw (TPR) — screw rotates; travelling nut carries the load. Useful for compact height or long screws supported at both ends.
- Keyed translating — anti-rotation key on the screw for external torque or side forces that would otherwise spin a free TP screw.
8. Multi-jack systems
If more than one lift point must stay level, size each jack as above, then design the shaft and miter bevel train so one motor drives all worms together. Layout patterns are covered in multi-jack synchronization layouts and on lifting systems.
Quick RFQ checklist
- Load per jack (kN) — static and dynamic; safety factor applied
- Stroke (mm) and free screw length for buckling check
- Speed target and duty cycle (on/off, cycles/hour)
- Hold without power? → Tr vs ball
- Upright / inverted / clevis / trunnion
- TP / TPR / keyed; limit switches; bellows environment
- Single jack or multi-jack plan (mm centres)
Related: Machine Screw Jacks · Screw Jack vs Hydraulic Cylinder · Contact