Selection
Servo versus standard hydraulic: where the energy saving comes from
Why a servo drive uses less electricity, how much saving is realistic rather than advertised, and the four things to check on a used servo machine that you would not check on a fixed-pump one.
The mechanism, briefly
A traditional hydraulic injection molding machine runs its pump at constant speed whenever the machine is on. The moulding cycle, however, needs full flow for only a small part of that time — during injection and clamp movement. For the rest of the cycle, and during the whole cooling time, the pump is still turning at full speed and the surplus oil is dumped back to tank over a relief valve. That energy becomes heat, which you then spend more energy cooling.
A servo drive replaces the constant-speed motor with a servo motor under closed-loop control. It turns the pump only as fast as the current moment demands, and close to zero during cooling. The saving is not a clever efficiency trick — it is simply the elimination of work that was never doing anything.
Where the saving is largest: long cooling times and thick parts. A machine with a 25-second cooling phase spends most of its cycle needing almost no flow, which is exactly the condition a servo drive exploits.
How much saving is realistic
| Compared with | Typical saving | Why the range is wide |
|---|---|---|
| Old fixed-pump hydraulic | 30–60% | Depends heavily on cycle profile and cooling share |
| Variable-displacement pump machine | 15–30% | That machine already throttles flow to some degree |
| Modern all-electric | Servo is usually behind | All-electric removes the hydraulic circuit entirely |
Quoted headline figures are usually measured on a favourable cycle. Work out your own number rather than accepting one: take your machine’s measured consumption per shot, multiply by shots per year, and apply your actual industrial tariff. In markets with expensive electricity the payback on the servo premium is often well under two years; where power is cheap and utilisation is low, it can be much longer.
Energy is not the only difference. A servo machine is also quieter, runs cooler oil — which extends seal and oil life — and holds injection and clamp positions more repeatably, which shows up as tighter part weight consistency.
What to check on a used servo machine specifically
- Servo motor and drive together. Ask to hear it start, stop and change speed. Squeal, judder or a delayed response points to the encoder, the drive tuning or bearing wear. This is the most expensive single component after the screw and barrel.
- Encoder and feedback. Position repeatability across consecutive cycles — look at the shot-to-shot consistency of both position and weight, not just one shot.
- Drive cabinet cooling. Servo drives are power electronics: check the filters, fans and whether the cabinet has been kept sealed. A drive that has been running in a dusty shop with the door open has had a harder life than its hours suggest.
- Parts availability for the drive. Confirm the exact drive model and check it is still supported. A rebuild or replacement drive is a substantial fraction of the machine value.
- Oil condition and temperature. One of the servo machine’s advantages is cooler oil. Hot or degraded oil on a servo machine suggests it has not been running the way it should.
Do not pay a servo premium for a servo badge. Ask for the drive model, and for the machine to be run through several cycles while you watch the position and weight numbers. A servo machine with a tired drive is worth less than a healthy fixed-pump machine.
So which should you buy?
- Buy servo if the machine will run more than one shift, if electricity where you are is expensive, or if your parts have long cooling times. The premium is recovered in power alone.
- Fixed-pump is fine for low utilisation, jobbing work, a back-up machine, or where the purchase price is the binding constraint and power is cheap.
- Consider all-electric only where precision or cleanliness demands it — the used market for all-electric machines is thin and the service knowledge is less widely available.
Keep reading
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See a servo machine run
Book a video inspection and watch the drive start, stop and change speed, with the position and weight readings on screen.