What is a servo press?

A servo press uses servo motors for precise force and position control during forming operations.

How do I calculate servo press tonnage?

Use our free tonnage calculator at servopress.club/tonnage-calculator.html

Tonnage nameplate vs actual capacity - what's the real number?

hank_diecast

The nameplate on our press says 200 tons but our tonnage monitor shows we can't actually hit that. What gives? Is the nameplate lying?

**Rated Tonnage vs Available Tonnage
**
The rated tonnage is the maximum force at a specific point in the stroke, usually 5-10mm before BDC. At other positions the available force is significantly lower: at 25mm above BDC typically 60-70% of rated, at 50mm above BDC typically 30-40%, at 100mm above BDC typically 15-25%.

For deep drawing where forming force is applied over a long stroke, check the force-stroke curve, not just the nameplate.

**Continuous vs Peak Tonnage
**
Peak tonnage is the maximum force for a single stroke. Continuous tonnage is the force sustainable at rated SPM without overheating. The continuous rating is often 60-70% of peak. For high-volume production size your press so required forming force is 70% or less of the continuous rating.

**Drive Mechanism Differences
**
Eccentric drive: force curve follows a sine function, maximum force near BDC. Knuckle joint: force curve is much flatter, better for deep drawing. Direct drive: force is constant throughout the stroke.

**Checking Actual Capacity**

Calculate required forming force at each point in the stroke. Overlay on the press force-stroke curve. Verify press force exceeds required force with at least 20% margin at every point. Check energy requirement against motor energy rating.

sarahJ_mfg

This is so important. We spec'd a 200T press for a job calculated at 180T. Turns out the 200T rating was at 10mm above BDC, and our forming happened at 25mm above BDC where available force was only 160T. Press was overloading on every stroke. Had to re-engineer the die to reduce force. Expensive lesson.

jim_apps

The torque curve vs force curve distinction trips up a lot of engineers. On a crank-type servo press, force = torque ? mechanical advantage, and mechanical advantage changes with crank angle. At TDC and BDC the mechanical advantage approaches infinity (theoretically) but the motor torque is the limit. Always ask the manufacturer for the actual force-stroke diagram.

carlos_mx

The 200T-at-10mm thing is painfully common. Here's what I tell every engineer specifying a servo press: ask the manufacturer for the FULL force-stroke curve, not just the nameplate number. Every reputable OEM will provide this. Plot your process force requirement on the same graph and make sure you have at least 20% margin at EVERY point in your working stroke, not just at the rated point. Another thing the nameplate doesn't tell you: the force rating is usually at a specific SPM. Run faster and available force drops because the motor has less time to develop torque. We tested a 300T press and found it could only deliver 240T at 30 SPM vs the rated 300T at 15 SPM. That's a 20% reduction that's nowhere on the nameplate. If you're running high-speed production, ask for the force-vs-SPM curve too. And one more gotcha: continuous vs intermittent rating. Some presses are rated at peak force for intermittent duty. If your process needs full tonnage on every stroke continuously, you might be thermally limited before you're mechanically limited. The motor overheats before anything breaks.

derek_tooling

The nameplate vs actual capacity question gets messy fast because there are at least four different numbers on the data plate and people use them interchangeably.

The number on the front of the press is the *rated tonnage at rated stroke position*. For a typical mechanical that's at 6mm above BDC. For a servo it's whatever the OEM tested at, often BDC or 1mm above. These are not equivalent ??comparing a servo's BDC rating to a mechanical's 6mm rating overstates the servo by 10-20%.

The second number is *continuous tonnage* vs *peak/intermittent tonnage*. Servo presses especially have a meaningful gap here. A 200-ton servo might have 200 ton continuous and 240 ton peak for short-duration draws. Run blanking at peak continuously and you cook the drive. The drive thermal model is what you're really pushing against, not the frame.

Third is *eccentric load capacity*. Most data plates list the centered tonnage and bury the eccentric derate in the spec sheet. For typical C-frames, an off-center die can derate the working tonnage by 30-50%. We've seen people specify a press for a die that lives in the eccentric-load derated region without realizing it.

Fourth is *stroke-position curve* (the cam curve on mechanicals, the velocity-torque envelope on servos). Tonnage available 25mm above BDC on a mechanical is a fraction of the rated number. On servo you can hold rated tonnage further up the stroke, which is one of the genuine technical advantages, but only if the drive is sized for it.

Practical advice: when sizing a press, work backward from the highest-tonnage operation in the deepest eccentric position you'll ever run, derate by 20% for safety, then check that against the *continuous* rating, not peak. If that math doesn't work, you're buying the wrong size press.