Ordering Linear Motion Components: A 5-Step Buyer's Checklist

If you're a design engineer, this checklist probably isn't for you. You live in this world. This one is for the rest of us—the people who end up ordering linear motion components without a mechanical engineering degree.

I manage purchasing for a 120-person automation company. Three locations, roughly $400K a year in component spend across 12 vendors. I report to both operations and finance. When our engineers need shafts, bearings, or motors, the purchase order lands on my desk. Over the past five years, I've built a simple checklist that keeps us from expensive surprises. It took a few painful mistakes to get here.

Step 1: Write the shaft spec down before you call anyone

Sounds obvious. It's not. The most common mistake I run into—and made myself—is ordering by brand name alone.

“Thomson linear shaft.” That was the entire spec on a $3,000 order once. I still kick myself over it. No diameter. No length. No tolerance class. The box that arrived couldn't fit into any of our assemblies. $3,000 of inventory, completely useless.

Before you send a PO for a Thomson linear shaft, write out:

• Diameter, in the same units as the bearing you're pairing it with
• Length—and specify cut-to-length if that's what you need
• Tolerance grade (g6 is common for shafting; h6 when you need tighter control)
• Hardness and surface treatment (hardened and ground, chrome-plated, etc.)

Writing it down on paper forces you to check it twice. And if you're searching distributor catalogs, the Thomson linear shaft family is usually listed under “thomson-linear”—don't let that trip you up like it did me.

Step 2: Match the bearing to the shaft tolerance, not just the diameter

Small ball bearings look like commodities. Honestly, many applications treat them that way. But the tolerance pairing between the bearing bore and the shaft is where the failures live.

Say you order a 12mm shaft. The nominal bearing bore is 12mm too. An actual bearing may only accept 11.992–12.000 mm. If your shaft measures 11.984, the fit is too tight. The assembly struggles, runs hot, and fails. We caught that one before it shipped. Barely.

ISO 10285 defines key dimensions and tolerance classes for linear motion bearings. But you shouldn't need to be an ISO expert to order parts. What works in practice: tell your supplier the exact shaft you're using and ask them to verify the fit. A supplier who can't answer that question is probably not the right one.

Step 3: Lock in the motor type early

Motor selection drives everything downstream—mounting, coupling, drive, control. Changing it late is a rebuild, not an adjustment.

For continuous running, a brushless DC motor is usually the right answer. They run cool, last a long time, and have no brushes to replace. For precision positioning, you may need a servo. For simple A-to-B moves, a stepper often wins on cost.

I went back and forth between a BLDC and a servo for a customer's conveyor for two weeks. The servo gave us closed-loop accuracy we didn't need. The BLDC was half the price and plenty sufficient. My gut said BLDC, but I kept second-guessing myself. Finally, I asked one of our senior engineers one question: “Continuous running, or positioning moves?” Continuous. Done. The entire decision took one sentence.

Step 4: Read the VFD output current, not just the horsepower label

This might be the most searched topic on this list: what size vfd for 5hp motor?

The rule is simpler than people expect. Match the VFD's rated output current to the motor's full load amps (FLA). The horsepower label is just a rough sizing shorthand.

For a NEMA design B, 5 HP motor at 460V, three-phase, FLA is about 7.6A. A standard 5HP VFD at 460V outputs around 8A. So yes, a 5HP VFD on a 5HP motor works, as long as the motor is 460V.

At 230V, the same 5HP motor draws about 15.2A. Some 5HP-rated VFDs at 230V output 15.2A; some output less. Look at the output current rating on the spec sheet. That's the number that counts. The “5HP” on the box is shorthand, not a spec. We undersized a drive once and got overcurrent faults within the first week of commissioning.

Step 5: Check the actuator stroke AND the duty cycle

Actuators come as a complete unit, which makes it tempting to treat them as turnkey. Two things get missed:

• Stroke length vs. retracted length—not the same dimension
• Duty cycle rating—how long it can actually run

If an actuator is rated for 10% duty cycle, that usually means one minute on, nine minutes off. If your process runs 12 seconds on and 8 seconds off, that's a 60% duty cycle. The actuator will overheat. It will fail. And it will do it on-site, in front of the customer.

This is where Thomson linear motion engineering makes a difference—good brands give you honest duty cycle numbers and guidance on how to read them. Others say “intermittent” and leave you guessing. Buy from someone who gives you the number. The math is your job afterward.

Mistakes I keep seeing (and paying for)

Saving $0.80 per bearing. We sourced cheap small ball bearings once. $0.80 cheaper per unit, 500 units, saved $400. Twelve failed in the field within six months. Rework, freight, customer calls, trust damage. $400 of savings against roughly $4,000 of cost. The quality of parts in a machine shapes the customer's perception of the machine itself. A noisy bearing sends a message, and it's not the one you want.

Assuming the datasheet describes the actual part. The spec said hardened and ground. The parts we received measured correctly but wore out three times faster than expected. A simple hardness check, ten seconds, would have flagged it.

Not asking the supplier to confirm before manufacturing. One line in the PO: “Confirm all specs before production.” It costs nothing and catches real errors. The suppliers who won't agree to it? That's the filter working as intended.

After five years and a few hundred orders, I've come to believe the “best” component is context-dependent—it depends on your application, your duty cycle, and your budget. But this checklist keeps me from the dumb mistakes. The ones that cost time, money, and credibility. Use it. Skip the expensive part.