Linear Guide Preload — Z0, ZA, ZB and When to Use Each

Preload is the internal clearance between the balls and the raceway. A guide with zero preload has a small gap; a preloaded guide has the balls pressed against both raceways, so the block does not move until the load reverses direction. It is a stiffness setting, not an accuracy grade, and the two are chosen separately.

What preload actually does

With clearance, the block shifts slightly every time the load reverses, which shows up as a step in the part or as chatter at the end of a cut. Preload removes that shift by keeping the balls in contact on both sides. The cost is friction and heat: the same block with ZB runs stiffer but warmer and needs more torque, so preload is chosen for the load case, not for the general impression of quality.

Preload does not raise the rated load. The dynamic load C printed in the catalogue is the same for Z0, ZA and ZB. What changes is the rigidity and the amount of heat at speed. A common mistake is to order ZB on a light axis to “make it more accurate” — the axis then runs hot, the grease ages faster, and the step the customer was chasing does not go away because the real problem was the structure, not the block. Position and clearance are separate choices: see the accuracy grades page for the other half.

The three preload grades

The grades are defined by the preload force as a fraction of the rated dynamic load C of the size. Because C scales with the size, the same letter means a different force on a 15 mm block and on a 45 mm block.

Z0 at the low end is not a defect: on a vertical axis the load keeps the block seated, so clearance does not show. On a horizontal axis with a reversing cut, Z0 shows up as a witness mark at every reversal.

Interchangeable blocks stop at ZA

Blocks supplied as interchangeable units cover Z0 and ZA, with accuracy C, H and P. ZB, like SP and UP accuracy, is supplied only as a matched block-and-rail set: the pair is lapped together so the preload is set against that rail, and a block from one set cannot be moved to another rail without losing the setting. This is why a quotation changes shape at ZB — the rail length has to be ordered with the blocks.

Preload or one size up

When an axis is not stiff enough, the temptation is to increase preload. Check the load first: if the working load is above roughly a third of C, the right move is the next size, not ZB. On a 25 mm block ZA gives 0.05–0.07 C of preload, while the 30 mm block with ZA has a higher C to start with and a larger ball complement; the bigger block is stiffer under load and cooler at speed than the smaller one pushed to ZB. Preload is the adjustment for play, not for capacity.

The same logic applies to accuracy. A guide ground to P on a frame that is not straight will follow the frame, and no preload will correct it. If the measured error follows the rail, look at the mounting surface before changing the grade.

Checking preload in service

Preload cannot be read off the block by eye. In service it shows up as starting torque: a preloaded block takes noticeably more force to move from rest than a Z0 block of the same size, and that difference is what a technician feels when sliding the carriage by hand before the drive is connected. If a ZA axis moves freely with no drag at all, either the preload is gone or the block is a Z0 fitted by mistake. If it is hard to move and warms up within minutes of running, the preload is higher than the application needs or the rail is not straight.

Because the setting lives in the matched pair, replacing a single block on a ZB axis means replacing the set. On Z0 and ZA axes an interchangeable block can be fitted to the existing rail, which is why most maintenance spares are ordered at ZA.

What to tell us when ordering

Give the model, the accuracy grade and the preload, and say whether the axis is horizontal or vertical and whether the cutting force reverses. If the preload is still open, describe the load and the overhang and we will recommend a grade rather than default to ZA. For ZB, send the rail length with the block order so the set is lapped as a pair.

Sizes and models this applies to

The rules above are the same at every size; the numbers differ. These pages carry the current figures for the sizes that come up most: HGH15CA, HGH25CA, HGW25CC, HGL30CA and MGN12. Series pages give the full range: HG series and miniature series; the block index is on linear guide blocks.

FAQ

Examples with real values: HGH25CA (26.48 kN), MGN12 (preload ZF / Z0 / Z1 on the miniature series). Back to technical reference.