When customers buy miniature linear guides (such as MGN series) for the first time, one of the most common concerns is: “The block feels loose on the rail, is the guide out of tolerance?"

In many cases this feeling comes from how the guide is being checked, not from a real quality problem. This article explains what “very light preload" means, why you may still feel some movement, and when you should consider a different preload level.

A frequent message from end users looks like this:

• “The block has obvious play on the rail."
• “When I rock the block by hand, I can feel clearance."

This feedback often appears after the customer receives a miniature guide like MGN12H1R300Z0C and tests it by hand, before installation on the machine.

Take the model MGN12H1R300Z0C as an example. It can be broken down as follows:

• MGN12H – 12 mm miniature linear guide, long type block type
• 1R300 – 1 pc rail of 300 mm length
• Z0 – very light preload (almost zero clearance)
• C – normal accuracy grade
• Material: Alloy steel

A common misunderstanding is: “Z0 means it is a loose, clearance type, that’s why it feels shaky."

In reality it is the opposite: Z0 is a very light preload level, designed to be close to zero clearance while still keeping friction low and installation more forgiving than heavy preload types.

If the block is moved by hand in free condition (rail not mounted, no table attached), some movement can usually be felt even with a very light preload.

In many cases the user:

• holds the rail in the air or on a soft surface,
• grips one corner of the block with fingers,
• rocks the block up/down or left/right with some force.

The motion observed here is mainly:

• elastic deformation of balls and raceways under side load, and
• small angular deflection in the rail, block and even the operator’s hand.

Visually it can look like a “gap", but in most cases it is simply elastic movement multiplied by the leverage effect, not a large free clearance.

The intention of the Z0 preload level is to:

• provide basic stiffness and positional accuracy,
• keep running resistance low,
• offer better tolerance to minor mounting errors.

Therefore it will never feel as “rock solid and locked" as a heavy preloaded guide. If someone expects absolutely no perceptible motion in any direction, even a small elastic movement may be judged as a defect, although it is normal for Z0.

While some movement by hand is normal for a very light preload, there are cases where further inspection is necessary:

• Rail and block are not originally matched
  Mixed sets, wrong models or random swapping between rails and blocks can completely change the preload condition.
• The block has been removed from the rail
  If the block has been taken off the rail and reinstalled, balls may be lost, contaminated or misaligned, resulting in true clearance.
• Visible damage or deformation on raceways
  Impact, dents, burrs or serious corrosion on the raceway can affect contact and preload.
• Excessive measured play after proper mounting
  Normal Z0 preload may show minimal elastic movement. If side play is clearly visible and can be measured as large (for example clearly above the expected tolerance) even after correct installation, the guide should be evaluated as a potential defect.

Some applications require a block that feels absolutely tight, with no noticeable play in any direction when installed. In such cases, a higher preload level, such as Z1 preload, can be considered.

Compared with Z0, a Z1 preloaded guide will:

• feel clearly tighter when moved by hand,
• offer higher stiffness and less elastic movement under load,
• provide a feeling much closer to “zero play".

However, this comes with an important requirement: the mounting base must be machined very flat and parallel. With higher preload:

• any deviation in flatness or parallelism is magnified,
• binding can occur at certain positions along the stroke,
• abnormal noise and local wear become more likely.

In short:

• For maximum rigidity and almost no perceptible play → consider a Z1 preloaded type and ensure a high-quality mounting surface.
• For smoother motion and better tolerance to mounting errors → a Z0 very light preload miniature guide is often the more practical choice.

To evaluate whether the observed movement is normal or not, the guide should always be checked in a condition close to real use. A simple procedure is:

1. Fix the rail on the machine base
   Mount the rail on a rigid, machined reference surface and tighten all screws to the specified torque.
2. Connect the block to the table or carriage
   Install the table, carriage or working platform on the block as in actual operation.
3. Move the axis through the normal stroke
   Manually move the axis and feel for binding, roughness or abnormal noise throughout the stroke.
4. Measure side play if necessary
   If higher precision is required, use a dial indicator to measure side movement at the table under specified load.
5. Record and share data
   If there is still clear free play after proper mounting, record a short video and the measurement results, then send them to the supplier for evaluation.