ZA vs ZB preload: what it actually changes on the machine
2026-11-06
# ZA vs ZB preload: what it actually changes on the machine
Most buyers treat preload as a checkbox next to accuracy grade. It is not. Preload is the internal clearance between the balls and the raceways, set at assembly. It decides how the block behaves before any external load is applied — and the wrong choice shows up months later as heat, drag, or chatter on a finish cut.
What preload physically is
In a ball-type linear guide, the balls can be left slightly loose (zero preload), or squeezed between the two raceway rows (positive preload). Squeezing removes the gap. That does three things at once: it increases rigidity, it increases friction, and it reduces life if you go too far.
The three standard levels
| Level | Preload amount | What it gives you | Where it belongs |
|---|---|---|---|
| Z0 | 0 – 0.02C | Free running, lowest drag | Light load, high speed, vertical axes with counterbalance |
| ZA | 0.05 – 0.07C (HG) | Rigidity without excess heat | Most machine tools, general automation |
| ZB | 0.10 – 0.12C | Maximum rigidity | Precision grinding, heavy cutting, matched set only |
The number is a fraction of the dynamic load rating C, not a force in newtons. On an HGH20CA with C = 17.75 kN, ZA means roughly 0.9 – 1.2 kN of internal preload force.
The detail almost everyone misses
The same letter means different force on different series. On our HG series, ZA sits at 0.05 – 0.07C. On the EG series it sits at 0.03 – 0.05C. If a customer orders "ZA" and you ship the EG block, the force he gets is not the force he specified.
Write preload with the series and the range attached: ZA (HG, 0.05–0.07C), not ZA.
How to choose
- Is the axis vertical? Use Z0 or a light ZA. Gravity already loads the block; preload on top just adds heat.
- Are you finishing a hard material? ZB, and only in a matched set with SP or UP accuracy.
- Is the machine fast? Higher preload raises drag torque, which raises motor load and temperature. On high-speed axes, ZA is usually the ceiling.
- Do you not know? ZA. It is the default for a reason.
When preload needs to be a matched set
ZB, SP and UP are only supplied as matched sets — block and rail ground together. That is not a commercial preference, it is geometric: at those preloads the clearance cannot be held if the block and rail are made as separate stock items. If you need to swap a single block later, you cannot, and you should plan spare sets instead.
A note on "upgrade the preload instead of the size"
The common instinct when a machine is not rigid enough is to raise preload. It usually does not work. Going from ZA to ZB on the same size buys you a little stiffness and costs you life and friction. Going up one size — HGH20CA to HGH25CA — buys about 49% more dynamic load rating and a thicker rail, for roughly the same friction per unit load.
Raise the size first. Raise the preload only when the geometry is already fixed and you cannot fit a bigger block.
What to send with an enquiry
- Model number and preload level, written as
HGH20CA, ZA (HG, 0.05–0.07C) - Accuracy grade — C, H, P, or matched set SP / UP
- Whether it must be interchangeable with an existing rail
- Axis orientation and the expected load per block
Get those four right and the quote matches the machine the first time.