Why Choosing a Small Bearing Is Not Always Simple

A miniature bearing can look like a very simple part.

Take the S681X, for example. Its outside diameter is only 4 mm. The S681XZZ has the same 1.5 mm bore and 4 mm outside diameter, but the bearing width is different.

For a buyer, this may look like a small detail.

In an actual assembly, it can matter.

We often see miniature bearings ordered by model number only. This works when the model is already confirmed. But when a customer is replacing an old bearing or developing a new product, it is better to check the dimensions and bearing design before placing the order.

S681, S681X and S681XZZ

These three models are easy to mix up.

They are all small bearings, but the dimensions are not the same.

The S681X and S681XZZ are the ones most likely to cause confusion. Both have a 1.5 mm bore and a 4 mm outside diameter.

The width is different.

That difference needs to be checked against the available space in the housing.

The suffix also tells you something about the bearing. S681X is an open version, while S681XZZ has metal shields.

So when sending an RFQ, writing only "S681" does not give the supplier enough information to know which version you need. 

Why Are Miniature Bearings Used?

The main reason is usually space.

Some small motors, instruments, rollers and compact mechanisms do not have enough room for a normal-size bearing. A miniature bearing allows the shaft and surrounding parts to stay small.

But a smaller bearing also leaves less room for mistakes.

A few tenths of a millimeter can affect the fit. The shaft, housing and bearing width all have to work together.

The bearing also needs to handle the actual speed and load of the application.

For this reason, we normally recommend checking the complete assembly instead of choosing a bearing from the shaft diameter alone.

Open or Shielded?

This is another point worth checking before an order.

An open bearing leaves the rolling elements exposed. It can be used when the bearing is installed inside a clean or protected assembly and the application calls for this configuration.

The S681XZZ uses ZZ metal shields.

The shields help reduce the entry of dust and other contamination from the surrounding area. They also help keep the grease inside the bearing.

That does not mean a ZZ bearing is waterproof.

If the equipment is exposed to water, humidity or other harsh conditions, the seal design and bearing material should be considered together with the application.

Choosing a shielded bearing simply because it sounds more protective is not always the right approach.

Stainless Steel Is Another Choice to Consider

The S681 series on our product page is made with stainless steel rings and balls. 

Stainless steel is often considered for equipment that may see moisture or a higher risk of corrosion.

This can be useful for small assemblies where the bearing is difficult to replace after the product has been built.

There is one point that buyers should keep in mind, though.

Not every stainless steel grade has the same properties.

If corrosion resistance is important to the project, it is better to specify the material grade instead of simply writing "stainless steel bearing" on the purchase order.

Installation Can Be More Important Than the Bearing Size

Installing a 4 mm bearing is not the same as installing a 40 mm bearing.

There is very little room for error.

The shaft and housing should be checked before installation. The fit should not be excessively tight unless the application requires it.

When pressing the bearing into a housing or onto a shaft, the installation force should be applied to the correct ring.

For example, when fitting the inner ring onto a shaft, the force should be applied to the inner ring rather than being transferred through the balls.

This is a basic point, but it is easy to overlook with miniature bearings.

A bearing can leave the factory in good condition and still develop noise or rough rotation because of poor installation.

What Information Should Be Included in an RFQ?

For standard replacement bearings, the model number and quantity may be enough.

For a new project, more information is helpful.

For example:

Model: S681XZZ
Quantity: 5,000 pcs
Application: Small motor
Speed: 20,000 rpm
Environment: Indoor
Lubrication: Grease
Requirement: ZZ shielded

If you have an existing bearing, a drawing or sample is even better.

This allows the supplier to check the basic dimensions and configuration before production.

It also reduces the chance of receiving a bearing that looks correct on paper but does not fit the assembly.

What Should You Check Before Buying 10,000 Pieces?

For a new supplier or a new bearing specification, we would normally suggest starting with samples.

The important checks are quite simple:

Does it fit the shaft?

Does it fit the housing?

Is the width correct?

Does it rotate smoothly?

Is the noise acceptable?

Does it work at the required speed?

Is the shield or seal correct?

Does it perform normally after assembly?

Once the sample is approved, the approved specification can be kept as the reference for the production order.

This is especially useful for OEM customers who plan to reorder the same bearing later.

The Smaller the Bearing, the More Carefully It Should Be Checked

Miniature bearings are small, but they are not necessarily easier to source.

With S681, S681X and S681XZZ, the differences can be found in the dimensions and configuration.

A bearing with the wrong width may not fit.

An open bearing may not provide the protection required by the application.

A material that is not suitable for the environment may lead to corrosion problems later.

And even the correct bearing can be damaged during installation.

So before placing a bulk order, it is worth spending a little time checking the actual bearing specification.

S681X or S681XZZ?

It depends on the assembly.

If you need an open 1.5 × 4 × 1.2 mm bearing, S681X is the relevant version.

If you need the same bore and outside diameter with a wider, ZZ-shielded configuration, S681XZZ is the option to check.

If you are not sure which one matches your existing bearing, send the dimensions or a sample to the supplier.

For miniature bearings, that is usually faster than trying to identify the part from a partial model number.

Final Note

A miniature bearing may only be a few millimeters in size, but the specification still needs to be exact.

When sourcing S681, S681X or S681XZZ, check the bore, outside diameter, width and bearing type. For a new application, also provide the speed, load, environment and lubrication requirements.

The more complete the information is at the beginning, the less likely there is to be a problem after production.

For a small bearing, getting the details right is often more important than getting the bearing itself.