Ball Screw End Machining and Support Bearings: The Part Most Buyers Forget

The lead decision creates an RPM problem you now have to solve

In the last article, we talked about 5mm vs 10mm lead. The basic takeaway was:

• Smaller lead = more thrust, higher RPM for the same speed

• Larger lead = more speed per RPM, less mechanical advantage

Once you’ve chosen the lead, you know the screw RPM required for your target speed. Now you have to make sure the screw can actually run at that RPM without vibrating, whipping, or destroying the bearings.

That’s where support arrangement and end machining come in.

A 16mm screw running at 3000 RPM on a 500mm span is usually fine with standard supports. The same screw running at 3000 RPM on a 1500mm span is a different situation entirely. The critical speed drops, and the support bearings have to be much more rigid.

So the lead choice from the previous article leads directly to this one: what RPM are you actually asking the screw to run at, and can your support arrangement handle it?

Fixed end vs floating end: what they actually do

Most ball screw assemblies use one of three support arrangements:

Fixed–Floating

One end is fixed. It has bearings that can take axial load in both directions—usually a pair of angular contact bearings or a preloaded bearing block. The other end floats. It can slide axially inside its housing to allow for thermal expansion.

This is the most common arrangement for general CNC axes.

Fixed–Fixed

Both ends are fixed. This gives higher stiffness and higher critical speed, but it requires pre-tensioning the screw to compensate for thermal expansion. If you don’t pre-tension correctly, the screw can bow when it heats up.

This is used on longer screws or high-precision machines.

Floating–Floating

Both ends float. The screw is only located by the nut. This is rarely used on machine tools because there’s nothing to control the screw’s axial position. You might see it on simple transfer mechanisms where accuracy doesn’t matter.

The important thing to understand is that the fixed end carries the axial load. When the nut pushes or pulls the table, the reaction force goes through the screw and into the fixed-end bearings. If those bearings are undersized or poorly mounted, the screw will deflect under load, and you’ll lose stiffness.

The floating end just keeps the screw from sagging and vibrating. It doesn’t carry axial load. But it still matters for critical speed and vibration control.

Why the floating end is not just a “free” end

I’ve seen customers treat the floating end as an afterthought. They use a simple radial bearing, or even a bushing, and assume it’s fine.

On a short, slow screw, maybe it is.

On a long screw running at moderate to high RPM, the floating end becomes important for controlling vibration. If there’s too much clearance in the floating end support, the screw can whip. The vibration increases with speed, and at some point it becomes destructive.

I’ve seen screws that ran smoothly at 1000 RPM but became unstable at 2000 RPM. The fixed end was fine. The floating end was allowing too much movement.

The fix wasn’t a new screw. It was a better floating end support with less clearance and better rigidity.

So when you’re selecting support bearings, don’t just look at the fixed end. The floating end needs to be matched to the screw length and operating speed too.

BK/BF and EK/EF: what the codes mean

If you’ve looked at ball screw support blocks, you’ve seen these codes.

BK is a fixed-end support block. It contains angular contact bearings and is designed to take axial load.

BF is a floating-end support block. It contains a radial bearing and allows the screw to slide axially.

EK and EF are similar, but with a different mounting flange shape. EK is fixed, EF is floating. They’re often used on smaller screws or where space is tight.

The codes are standardized, but the actual dimensions vary by manufacturer and series. A BK15 from one supplier might not have the same mounting hole spacing as a BK15 from another.

So when you’re ordering, don’t just ask for “BK15.” Ask for the drawing and confirm the dimensions match your screw end machining and your mounting surface.

I’ve had customers order BK/BF blocks that looked right in the catalog, only to find the mounting holes didn’t line up with their existing bracket. That’s a frustrating discovery when the machine is already apart.

End machining: where the screw meets the support

The end of a ball screw is machined to fit into the support bearing. That machining includes:

• The journal diameter where the bearing sits

• The shoulder that locates the bearing axially

• The threaded section for the locknut

• Sometimes a keyway or flat for torque transmission

• The overall length of the machined section

If any of these dimensions are wrong, the bearing won’t fit properly. If the journal is too small, the bearing will have play. If the shoulder isn’t square, the bearing will tilt. If the thread is the wrong pitch, the locknut won’t tighten correctly.

I’ve seen screws where the end machining was done to a different standard than the support block. The customer had to machine an adapter or order new blocks. That’s downtime and extra cost that could have been avoided by confirming the drawing before production.

So when you order a ball screw, always confirm the end machining drawing against the support block drawing. If you’re replacing an existing screw, measure the old one and send those dimensions to the supplier. Don’t assume that “SFU1605” automatically comes with the end machining you need.

Critical speed: the number that decides if your screw will whip

Critical speed is the RPM at which a rotating shaft becomes dynamically unstable. Below that speed, the screw runs smoothly. Above it, vibration increases rapidly and can damage the screw, bearings, and nut.

The critical speed depends on:

• Screw diameter

• Screw length between supports

• Support arrangement (fixed-floating vs fixed-fixed)

• Bearing rigidity

• Whether the screw is pre-tensioned

A larger diameter raises the critical speed. A shorter span raises it. A fixed-fixed arrangement raises it compared to fixed-floating.

So if your application requires high RPM—either because you chose a small lead or because you need high linear speed—you have to check the critical speed.

If the required RPM is above about 80% of the calculated critical speed, I’d look at alternatives:

• Larger diameter

• Larger lead

• Shorter span

• Fixed-fixed support

• Pre-tensioning

Ignoring critical speed is one of the most common reasons a ball screw fails early or never performs as expected.

Alignment: the silent killer of ball screw performance

I’ve said this in other articles, and it applies here too: alignment matters more than most people think.

A ball screw that isn’t parallel to the linear guides will have uneven running resistance. The nut gets pulled sideways. The motor current goes up. The screw and nut wear faster.

I’ve seen machines where the screw was perfectly straight, the bearings were high quality, and the end machining was correct—but the axis still felt tight. The problem was that the screw centerline and the guide centerline weren’t parallel. The nut was fighting the guides through the whole stroke.

Checking alignment isn’t complicated. Mount the screw and the rails, then run a dial indicator along the screw while moving the carriage. If the needle moves more than a few hundredths of a millimeter, you have a problem.

This is especially important when you’re replacing a screw on an existing machine. The old screw might have been aligned to the old rails. A new screw with slightly different end machining might not sit in the same position. Always re-check alignment after installation.

Thermal expansion: why the floating end exists

When a ball screw runs, it heats up. Friction in the bearings, the nut, and the screw itself generates heat. The screw expands. On a 1000mm screw, a 5°C temperature rise can add roughly 0.06mm of length. That doesn’t sound like much, but on a precision axis, it matters.

If both ends are fixed, the expanding screw has nowhere to go. It bows. That changes the preload, increases friction, and can cause vibration.

That’s why the floating end exists. It lets the screw grow without bowing.

But the floating end has to actually float. If the bearing is pressed too tightly into its housing, or if there’s debris in the fit, it won’t slide. Then the screw behaves like it’s fixed on both ends, and you get the bowing problem anyway.

So when you install a floating end support, check that the bearing can slide axially. It should move smoothly but without excessive play. A little grease on the sliding surface helps.

What to send your supplier before ordering

If you’re ordering a ball screw with end machining and support blocks, don’t just send a model number.

Send this:

• Screw diameter and lead

• Overall length

• Travel length

• Fixed end / floating end arrangement

• Support block model or drawing

• End machining drawing

• Motor connection details (coupling, pulley, etc.)

• Mounting orientation

• Maximum speed

• Acceleration

• Load and cutting force

• Duty cycle

A good supplier will check the end machining against the support block and confirm the critical speed for your application. If they don’t ask for these details, they’re just selling you a part, not solving your problem.

I’ve had customers send me a sketch on a napkin and a photo of the old screw. That’s enough to work with. What’s not enough is “SFU1605, 1000mm, quote please.”

A quick checklist before you install

Once you have the screw and supports, before you bolt everything down:

• Confirm the end machining matches the support block

• Check that the fixed-end bearings are preloaded correctly

• Check that the floating-end bearing can slide axially

• Verify the screw is parallel to the guide rails

• Check the coupling alignment

• Confirm the locknut is tight and the bearing is seated

• Rotate the screw by hand and feel for tight spots

• Run at low speed first and listen for unusual noise

If something feels wrong, stop and investigate. A ball screw that runs smoothly by hand will usually run smoothly under power. A screw that feels tight or rough by hand won’t magically fix itself when the motor is connected.

Final thought: the screw is only part of the system

Choosing the right diameter and lead is important. But the ball screw doesn’t work alone. It needs proper end machining, matched support bearings, correct alignment, and a mounting structure that doesn’t flex.

I’ve seen a perfectly good screw perform poorly because the floating end was wrong. I’ve seen a high-precision screw fail early because the alignment was off. I’ve seen a machine that couldn’t reach its rapid speed because the critical speed was never checked.

None of those problems were solved by buying a more expensive screw.

So before you order, look at the whole assembly. The screw, the supports, the end machining, the mounting, the alignment. If any one of those is wrong, the machine won’t perform the way you expect.

And if you’re not sure, send your supplier the drawing and the application details. A few minutes of checking before the order is much cheaper than discovering the problem after the machine is assembled.