Six-Step Checklist for Specifying a Thomson Saginaw Linear Actuator or Thomson Linear Ball Screw System

I'm an application engineer who has handled linear motion orders for 12 years. I've personally made—and documented—14 significant mistakes that totaled roughly $42,000 in wasted budget. This is the checklist I run before sending a purchase order now. If you're specifying a Thomson Saginaw linear actuator, a Thomson linear ball screw, or a motion axis powered by an ac induction motor, this is for you.

It's six steps. The first four take ten minutes. The last two keep me from spending money twice.

Step 1: Define the duty cycle before you look at the part number

In my first year (2017), I ordered a Thomson Saginaw linear actuator for a lift table based on peak thrust. The actuator handled the peak. The motor didn't. After five cycles, the thermal overload opened because I had ignored a 45% duty cycle at full speed. $2,300 in rework, plus a week of schedule pain.

Most buyers focus on peak load and completely miss the duty cycle. Why does this matter? Because heat kills actuators. A peak thrust number with no frequency, stroke, ambient temperature, and dwell time is just a brochure number.

Check these before you call it done:

• Peak thrust and holding thrust
• Stroke, linear speed, and acceleration
• Cycles per hour and dwell between moves
• Ambient temperature and enclosure
• Required life in cycles or hours

If you don't have all five, the quote isn't a quote. It's a guess.

Step 2: Know the difference between pitch and lead on a ball screw

This one cost me a full reorder in 2019. I specified a Thomson linear ball screw with a 10 mm pitch when I meant 10 mm of travel per revolution. The screw had two starts, so the pitch was 5 mm and the lead was 10 mm. I got exactly what I ordered: a screw that moved 5 mm per revolution. It worked, just at half the speed. That required a new screw and a redesign of the timing. $1,150 wasted.

Let me rephrase that so it sticks: lead is travel per revolution. Pitch is the distance between thread starts. If a screw has multiple starts, they are not equal.

Check: read the lead on the datasheet, not the pitch. If the part number doesn't say lead, call the manufacturer. I should add that this applies to every ball screw, not just Thomson.

Step 3: Check critical speed before you fall in love with a shaft diameter

This is the step most engineers skip, and it's the reason I include it. Everyone checks static load. Almost nobody checks critical speed. A long, unsupported screw can whip at an RPM far below its rated load limit.

On a 36 mm screw with 1,800 mm of unsupported length, the screw passed every load check, but at 900 rpm it vibrated violently. We had to add an outboard bearing hanger and re-machine the mount plate.

Check these numbers before you commit:

• Unsupported length between bearing supports
• Screw diameter and root diameter
• End bearing fixity (fixed-free, fixed-supported, fixed-fixed)
• Maximum operating speed as a fraction of critical speed

The manufacturer's critical speed chart is the authority here; use it. Not ideal, but workable if you catch it before metal gets cut.

Step 4: Match the motor to the full speed-torque curve, not just the nameplate

If your axis is powered by an ac induction motor, the nameplate torque is the continuous torque at one operating point, usually 60 Hz. It doesn't tell you what happens at 700 rpm under load. In 2021, I selected a 2 HP ac induction motor for an indexing table using only the nameplate torque. It ran fine at 1,750 rpm, then stalled at 700 rpm after ten minutes. $3,400 in redo plus a missed ship date.

Why does this matter? Because an ac induction motor's cooling fan is typically on the rotor shaft. At half speed, a standard fan delivers far less cooling air. If you're running through a VFD, you need to check the motor's low-speed torque rating.

What's a VFD? In short, it's a variable frequency drive that changes the frequency and voltage feeding an ac induction motor to control speed. It is not a magic torque box. Per IEC 60034-17, converter-fed AC motors need a thermal and torque check across the intended speed range, especially at low speed. Source: IEC 60034-17.

If the application needs full torque down to low speed, ask for an inverter-duty motor with separate forced ventilation, or oversize the motor so the nameplate torque is enough at the reduced cooling condition.

Step 5: Choose the servo motor encoder with the inertia ratio in view

A servo motor encoder is not an independent accessory. Its resolution and update rate have to work with your load-to-motor inertia ratio, drive tuning, and speed profile. I learned this on a rotary axis where I chose a high-resolution encoder trying to solve a coarse positioning issue. The axis oscillated anyway, because the real problem was the 20:1 inertia mismatch. The encoder was fine; my mechanical spec wasn't.

People think more encoder lines equal more stability. Actually, the inertia ratio sets the achievable loop gain; no encoder can compensate for an unstable mechanical load.

Check these values:

• Reflected load inertia referred to the motor shaft
• Ball screw inertia and coupling inertia
• Recommended inertia ratio from the drive manufacturer
• Encoder line count and maximum speed range
• Feedback type (incremental, absolute, BiSS, Endat, etc.)

If the inertia ratio is too high, fix the mechanics first. Then let the servo motor encoder selection support the motion profile.

Step 6: Verify VFD and motor compatibility before ordering

What's a VFD? I already said it: a variable frequency drive. But knowing what it is and knowing when it works with your motor are different things.

I once ordered a 480 V ac induction motor and a 460 V-rated VFD without checking the motor nameplate wiring diagram. The drive faulted on overvoltage during deceleration because the application had an overhauling load and no brake resistor. That lesson cost me $780 in parts plus a week of troubleshooting—on a system I failed to review carefully.

People think a VFD is universally compatible with every ac induction motor. The reality is that compatibility depends on voltage, current, cable length, insulation rating, and load profile.

Verify voltage range, motor FLA below drive continuous current, cable length within the drive manufacturer's IGBT spike limit, brake resistor for overhauling loads, and feedback type for the control mode. That last item is especially important if you're pairing a VFD with a servo motor encoder or an external encoder on the motor shaft.

Final reminders before you hit send

The six steps above are the structure, but three rules still catch our team:

1. Don't use the dynamic load rating as the continuous duty rating. A ball screw's C rating is based on a million revolutions. If your machine runs 400 rpm, eight hours a day, that million-revolution life goes fast. Use the life calculation or ask the manufacturer for the L10 number for your exact cycle.

2. Measure the existing system before you reuse a part number. If you're replacing a failed actuator, don't order by nameplate alone. Machines get modified, shims get added, and old markings lie. I should add that this is how I caught a 12 mm stroke mismatch on what looked like a standard replacement.

3. If the motor runs on a VFD, never assume self-cooling. The shaft-mounted fan on a standard ac induction motor moves less air at speed. If you need continuous torque below roughly half base speed, specify separately forced ventilation or an inverter-duty motor from the start.

Since I started using this checklist, our team has caught 47 potential errors in 18 months. That is the efficiency gain that matters: fewer reorders, fewer delays, and fewer late-night calls. Not exciting, exactly. But better than the $42,000 kind of learning.