A 7-Step Linear Motion Checklist: What I Check Before Specifying Actuators, Shafts, and Bearings

Before you place a linear motion order, spend ten minutes on this checklist. I've built it from 23 mistakes that cost about $38,000 in scrap, rework, and missed deadlines. There are seven steps. Follow them in order, or at least do Step 1, Step 4, and Step 6 before any vendor gets a PO.

Why I built this checklist

I'm a motion-control applications engineer, and I've been handling orders in the linear motion world for eight years. I've personally made—and documented—more significant mistakes than I'd like to admit. In my first year (2017), I made the classic mistake of matching an old ball screw replacement by pitch alone. It fit. It ran. But it was the wrong accuracy class, and the customer's positioning repeatability was never right. That error cost about $890 in redo plus a one-week delay.

Now I maintain our team's pre-order checklist. The goal isn't to make everyone perfect. It's to catch the errors that look fine on a screen but turn into expensive problems on a machine.

When I first started specifying linear actuators, I assumed a bigger frame always meant a better system. Three failed load tests later, I learned that frame size doesn't forgive wrong duty-cycle assumptions. That's the kind of thing this checklist is for.

The seven checks

Step 1: Check the shaft grade before you trust the straightness

A plain steel shaft is not automatically a linear shaft. For a precision linear guide system, the two numbers that matter most are straightness and surface hardness. I've used bar stock from a local machine shop for non-critical shafts. For critical axes, I order a Thomson linear shaft with a published straightness grade and a hard-chrome surface that doesn't turn into a rust streak after two weeks.

Don't assume the bearing block will fix a bent shaft. It won't. The bearing will just transfer the shaft's error into the carriage, and you'll spend days shimming something that should have been straight in the first place.

• Confirm the straightness tolerance in writing.
• Check surface hardness if the application has wear risk.
• Measure the shaft in the mounted condition, not standing in the corner.

That last one is the one I ignored the most. A shaft can sag under its own weight when it's stored vertically, and that's usually where my field dimension goes wrong.

Step 2: Size the linear actuator for dynamic load, not static load

Static load is the force the actuator can hold without moving. Dynamic load is the force it can handle while actually moving. You need the dynamic number. If you're replacing a legacy product and the drawing calls out a Thomson Saginaw linear actuator, don't just match the stroke and push force on the nameplate—check the catalog for the current equivalent.

I once ordered a replacement actuator assembly for a customer who gave me an old Saginaw part number. The stroke matched, the mounting footprint matched, but the dynamic capacity was too low for the cycle rate they were running. The actuator wasn't broken when it arrived; it just failed early because I sized it like a static jack.

I'd rather spend ten minutes explaining how dynamic ratings work than deal with a failed field retrofit later. An informed customer asks better questions and makes faster decisions.

Step 3: Match the motor to the inertia of the load

For a small ball screw, a NEMA17 stepper motor is often enough. But 'NEMA17' only tells you the size of the mounting face. It doesn't tell you the torque, the rotor inertia, or whether the motor can actually reach your target speed while the load is attached.

What I mean is that a NEMA17 stepper motor can look fine on the torque-speed curve when it's unloaded. The moment you couple it to a ball screw with a heavy table and a high acceleration rate, the rotor inertia mismatch can make the motor lose steps or stall. That's the part nobody talks about in the catalog.

• Find the load inertia reflected to the motor shaft.
• Compare it to the motor's rotor inertia.
• Check the torque-speed curve at your actual step rate or speed, not just holding torque.

If the ratio is above 10:1, you'll feel the pain sooner or later. On a prototype, it's a tuning issue. On a production machine, it's a crash.

Step 4: Look at the mounting surface before you pick crossed roller bearings

Crossed roller bearings give you high rigidity and low friction in a compact envelope. But they are not forgiving. They need a flat, clean mounting surface and proper preload. Most people ignore this step because it's not in the parts list.

I calculated the worst case once: if the base plate was off by a few microns, the bearing pack would twist during assembly and die early. I kept asking myself if the $400 faster-delivery option was worth the risk. It wasn't. We machined the surface, lapped the spacer, and the system worked.

If you're using crossed roller bearings on a slide or a rotary table, check the flatness with a precision straightedge before assembly. That's it. Simple. The bearing can't correct what the base did wrong.

Step 5: If you're using a VFD, plan for low-speed torque

People ask me how VFD control motor speed works, then they assume the drive can also hold a precise linear position. VFD control is great for speed regulation, but it's not a positioning servo. The drive changes frequency to change speed, and it changes voltage to keep the motor from overheating in the constant-torque range. Below a certain frequency, a standard AC motor loses torque and cooling.

In September 2022, I set up a linear transfer unit with a VFD and ran it at 3 Hz for too long. The motor overheated, tripped, and shut down a line. The VFD was doing exactly what it was told. The problem was my selection: I didn't account for continuous low-speed torque or forced cooling.

• Check the VFD's constant-torque speed range, not just the base speed.
• If you need position accuracy, use a closed-loop control with an encoder or a servo.
• If you need low speed for long periods, add external cooling or oversize the motor.

You can make a VFD work for a lot of linear motion applications. Just don't ask it to do something it wasn't designed to do.

Step 6: Verify interface dimensions before you place the PO

This step sounds obvious, but it's where I've lost the most money. On a 42-piece order, every single item had the right diameter, right lead, and right material. The mount flange was wrong. It was a $3,200 mistake, straight to rework, and it cost us a week of production time.

The fix is simple: get the actual mechanical drawing from your CAD library, compare it to the vendor drawing, and measure the mating part with calipers. Don't trust the part number alone. Part numbers change, revisions happen, and manufacturers modify flanges without shrinking the catalog photo.

On the legacy side, this is especially true. A Thomson Saginaw linear actuator that was designed in the 1980s may not match a 2025 equivalent in the fine details even when the main dimensions match. Documenting the interface dimensions before ordering prevents the fastest, most embarrassing failure mode in our industry.

Step 7: Write the lesson down in your own words

After the third rejected part in Q1 2024, I created our pre-check list. The most important change wasn't the technical review. It was writing down exactly what went wrong and why. When a mistake is only in your head, it evaporates under deadline pressure.

We've caught 47 potential errors in the past 18 months using this checklist. Some were small. A few were order-of-magnitude capacity mistakes. The simple act of putting the failure mode into writing made people stop and think before confirming the PO.

Final notes and common mistakes

Here are the patterns I still see when I review other people's linear motion orders:

• Skipping fatigue calculations on a linear shaft because 'it's just pushing light loads.'
• Choosing a NEMA17 stepper motor by frame size alone, without checking the inertia ratio.
• Using crossed roller bearings on a base plate that isn't flat enough for them to work.
• Assuming a VFD can hold a linear position accurately without an encoder or servo loop.
• Blindly cross-referencing legacy Thomson Saginaw part numbers without verifying the current catalog ratings.

This checklist won't make your system perfect. But it will catch the mistakes I've already made so you don't have to repeat them. The nice thing is that if you take ten minutes on each step, you'll know more about your system than the catalog shows. That's the point.

Source references: ISO 3408-1 for ball screw acceptance; NEMA MG 1-2021 for motor ratings; Thomson Linear published engineering ratings and application notes, as reviewed in Q1 2025.