Machine Screw Jacks: How to Spec a Joyce Worm Gear Jack

The load is up, the motor is de-energized, and nothing moves. Holding position with no power applied, no brake in the drivetrain, and no hydraulic circuit to leak down is why machine screw jacks keep getting specified into new equipment — damper drives, roll adjustments, maintenance platforms, antenna positioners.

But self-locking is a property of a specific model number, not of the product family. At HESCO, we help customers size Joyce/Dayton screw jacks across conveyor, packaging, steel, and process applications, and a handful of decisions drive most of those selections. Here’s what actually matters.

The short version: Joyce worm gear machine screw jacks run 250 pounds to 250 tons, in tension or compression, upright or inverted. Single lead WJ and RWJ models are self-locking; double lead DWJ and DRWJ models are not. Efficiency runs from roughly 34% down to under 9%, which is why torque, duty cycle, and heat drive the sizing more than the static rating does.

Start With Column Load, Not Nameplate Capacity

Joyce rates these jacks on static thrust capacity, but three things narrow the selection from there — and all three can push you to a larger model than the load alone suggests.

• Column load. In compression, the limit is the lifting screw’s buckling capacity at full rise, not the nameplate. A 5-ton jack at 8 inches of rise and the same jack at 60 inches are not the same device. Check Joyce’s column load charts against your load at maximum extension.
• Travel speed. Speed comes from lead and gear ratio, and both are coarse steps rather than a continuum. If the required rate falls outside what the available ratios deliver at a sane input RPM, a double lead or ball screw jack is the better answer.
• Duty cycle. Heat builds between the screw and the bronze nut, and at 15–25% efficiency most of the input energy ends up there. Joyce is explicit that duty cycles must include periods of rest. A cycle that never rests is the clearest signal you’re looking at the wrong product family.

Gear Ratio Sets Torque, Speed, and Resolution at Once

The worm gear ratio is the most consequential number on the spec because it moves three things simultaneously. A higher ratio drops the input torque the motor must supply, slows travel, improves positioning resolution, and costs efficiency.

Torque figures are coefficients: multiply by the load in pounds, then add tare torque (20 in-lb on a WJ810). Source: Joyce/Dayton published specifications.

A WJ810 at its full 20,000 lb rating needs 1,240 in-lb to start and 880 in-lb once moving. Size the motor to the starting figure — one picked off the operating number will stall on the first cycle.

The ratio also sets positioning resolution, which is easy to miss. A WJ2510 needs 100 worm shaft turns per inch; the same jack at 8:1 needs 16. If closed-loop positioning is part of the design, choose the ratio with the encoder in mind, not just travel speed.

Read the Asterisks on Self-Locking

Single lead WJ and RWJ jacks hold position without a brake. Double lead DWJ and DRWJ jacks, which exist to double travel per input revolution, are not self-locking and may lower under load. The WJ500 miniature carries the same flag.

There’s a design trap here. A double lead jack buys speed and efficiency — DWJ810 runs 31.9% against 23.1% for the single lead — but it gives up the one property that distinguishes a machine screw jack. If your application is already carrying a brake motor, that advantage is gone, and the comparison you should be running is against a ball screw jack instead.

Jack Design: What Restrains Rotation

• Translating (S) is the default. Something external must prevent the lifting screw from rotating; tying two or more jacks to a common load usually handles it.
• Keyed for non-rotation (K) puts a key fixed to the housing into a keyway milled the length of the screw, so it translates without rotating on its own. Use it when the load can’t restrain the screw — for example when the jack rises to meet the load. Available on WJ500 and larger.
• Keyed for traveling nut (KFTN) inverts the arrangement: a fixed-length screw rotates and a flanged nut carries the load along it. No extending screw means no protection tube, which makes this the choice for flush-mount installations and tight envelopes.

Anti-Backlash for Reversing Loads

Backlash is free play between the screw and nut, and it matters in reversing-load applications where screw position is critical — rolling mill roll adjustment is the classic case, and wind buffeting on solar and antenna positioners does the same thing. Joyce offers three designs, all clamping two independent nut halves against the screw threads.

All three carry full jack capacity under static load. Source: Joyce/Dayton published specifications.

Two practical notes. Adjusting within the recommended range doesn’t increase the torque needed to move the load — but setting backlash tighter than recommended raises torque significantly and accelerates thread wear. And because the device compensates for nut wear as you re-adjust it, machine screw jacks have a maintainable, extendable service life. Anti-backlash devices cannot be used on ball screw jacks.

Options Worth Settling at Spec Time

• Feedback: encoders at 200 or 1024 PPR quadrature, with absolute and stainless versions. Geared potentiometers as 0–10 V or 4–20 mA, with IP65 available.
• Motors and mounts: NEMA adapters MMA (56C) through MMD (210TC) on 2- to 20-ton jacks; servo mounts on 2- to 10-ton. If you’ll vary frequency for a soft start, an inverter duty motor may be required — catch that now, not at commissioning.
• Envelope: protective boots and screw stops can both increase closed height. Note too that rise is travel in inches, not physical screw length — a distinction that has caused more than one clearance problem on a first build.

The Bottom Line

Get the ratio right and you’ve settled torque, speed, and resolution in one decision. Check column load at full rise rather than nameplate capacity. Respect the duty cycle, because the heat is real. And read the asterisks on self-locking — a double lead jack that needs a brake motor has already given away the reason you chose a machine screw jack.