How to Achieve Mechanical Synchronization Across Multiple Screw Jacks

Mechanical synchronization is one drive shaft tying every unit's input shaft together, with a single drive turning all of them. But it is not "guaranteeing they stay identical" — it is "holding the error within an acceptable band." The first is a promise; the second is engineering.

Five error sources sit behind that band: lead deviation, drive-shaft torsional deflection, coupling backlash, installation tolerance and uneven load. Torsional deflection is calculable and counter-intuitive: it accumulates along the shaft, the far unit lags in phase, and the lag scales with torque, shaft length and lead but falls with the fourth power of shaft diameter.

This article does not promise any synchronization accuracy figure.

The Three Ways to Connect Multiple Units

The choice is not "which is better" — the span and the unit count decide it. Where a shaft can be run, run a shaft; only where it cannot, go electrical.

Electrical synchronization is explicitly outside this article. It rests on control strategy, feedback devices and algorithms. What follows covers mechanical synchronization only: direct series and distribution gearbox in parallel.

Five Sources of Synchronization Error

The most important column in that table is the last one. It answers "why does it pass unloaded and lose synchronization once load is applied?" Of the last three sources, two vary with load — and the fifth is not merely pushed around by load, it *is* a function of load. No unloaded test can see any of them.

Calculating the Torsional Lag

This is the part that turns "the shaft twists a little" into something you can design against. Start with the angle of twist in a shaft segment:

φ = T · L / (G · I_p)

T is the torque transmitted through that segment, L is the length of the segment, G is the shear modulus of the shaft material, and I_p is the polar moment of inertia of the shaft section.

For a solid round shaft, the polar moment of inertia is:

I_p = π · d⁴ / 32

d is the shaft diameter.

Twist is an angle, and what the machine feels is how much less the far unit has travelled. Convert it through the screw lead:

Δs = φ · P / (2π)

P is the screw lead and Δs is the lagging displacement of the far unit.

Substituting the three into one another gives a single expression:

Δs = 32 · T · L · P / (2π² · G · d⁴)

That expression deserves a second look, because it splits synchronization error into five controllable quantities — and only one of them is raised to a power.

Four Design Levers

The combined expression reads off directly: error is proportional to torque `T`, shaft length `L` and lead `P`, and inversely proportional to the fourth power of shaft diameter `d`.

So there are four levers:

Why does shaft diameter come first and not last? Because of the fourth power. The other three levers improve the error linearly; shaft diameter improves it to the fourth power. That asymmetry makes it the cheapest single move in mechanical synchronization design — and the one most often skipped, because a slender shaft gets chosen on linear intuition.

The same expression reads in reverse. If a machine will not synchronize, look first at whether the drive shaft is too slender, rather than at the coupling or the control programme: checking the shaft diameter is usually the more effective of the three.

Commissioning: Three Measurements

However good the design calculation is, it has to be measured, because lead deviation, clearance and installation tolerance cannot be pinned down from drawings.

All three steps have to record position, not only the end point. Synchronization error rarely grows evenly; it usually peaks at some intermediate position, and a test that reads only the end of stroke will not find that peak.

Reading the Symptom: A Diagnosis Table

Field Checklist

At design stage:

1. Fix the unit count and the layout — direct series or distribution gearbox.
2. Calculate the torsional lag Δs for the worst shaft segment.
3. Check the shaft diameter first, for being too slender (the fourth-power relationship).
4. Confirm the unit loads are close; where they differ widely, calculate them separately.

At installation:

1. Measure base elevation and verticality for every unit.
2. Record the coupling type and its fit condition.
3. After the shaft is connected, bar the drive over by hand and confirm nothing binds.

At commissioning:

1. Run the full stroke unloaded, measure the height difference at several positions, and record the position of each.
2. Repeat at the same positions with load applied.
3. Measure the lost motion on reversal.
4. Tabulate the three sets of data as the baseline for later maintenance.

Common Errors and Scope

Common errors:

1. Assuming mechanical synchronization means the units will always match. It bounds the error; it does not remove it.
2. Measuring only the end point. The largest error often sits somewhere in the middle of the stroke.
3. Accepting an unloaded test as the delivery test. The error changes once load is applied.
4. Specifying the drive shaft too slender, picking it on linear intuition and ignoring the fourth-power relationship.
5. Treating the starting height difference from installation tolerance as a synchronization problem. That one is a levelling job.
6. Ignoring the reversal lost motion. A single-direction test cannot see backlash.
7. Forcing a shaft across a span too wide for it, and ending up with a shaft that is both long and slender.

Scope of this article:

• The layouts, the five-source analysis, the calculation chain and the commissioning method apply to mechanical synchronization of multiple units in the SWL series of worm gear screw jacks.
• No synchronization accuracy figure or accuracy grade appears in this article. Achievable synchronization depends on configuration, installation and measurement, and has to be confirmed on site.
• No numeric values are quoted for shaft diameter, shaft length, torque or lead; those follow from the calculation for a specific project.
• Electrical synchronization design is not covered here — control strategy, feedback devices and algorithms sit in another field.
• This article does not replace strength verification or whole-machine safety design for a specific project.

Data to Send for an Enquiry

Item 7 needs a particular note: the acceptable height difference is given by the process, not by the supplier. That single figure often decides the arrangement more than a model number does — where the requirement is tight, the answer is a heavier shaft or fewer units in series.

No unit count is prescribed here. The layout follows from the span and the loads; a habit is not evidence, and this article offers no statistic about what other projects do.

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 parameters SWL screw jack selection needs
• Industrial gearbox installation, lubrication and first-run checklist

Next Step

Send the ten items above to sales@chinfar.com. Items 3 (spacing between units) and 7 (acceptable height difference) matter most — with those two numbers the worst shaft segment can be sized.

To be explicit: this article does not promise any synchronization accuracy figure. Mechanical synchronization is a band that is calculated, built and then measured on the machine; where that band falls for your installation can only be established by calculation and by test.