<h3>Bearing on a shaft</h3>
<p>An 8 mm (ID, Internal Diameter) bearing should always fit on an 8 mm (OD, Outer Diameter) shaft. If it does not fit, one of the 2 is of a different size. Please use a caliper to determine the size of both.</p>
<h3>Bearing in a housing</h3>
<p>If it is the outer size of the bearing, i.e. fitting in a printed carriage or a bearing housing (e.g. SCxxUU), you should be aware of the different sizes of polymer bearings. E.g. there are RJMP-01-xx and RJ4JP-01-xx types of polymer bearings (xx denotes the shaft diameter, e.g. 08 for 8 mm shafts).</p>
<p>For RJMP-01-xx bearings (<strong>not</strong> compatible with LMxxUU bearings):</p>
<pre>
// Parameters for RJMP-01-xx (shaft size xx, all dimensions in [mm])
// Designation  d1  d2  B   B1      s   dn
//-------------------------------------------
// RJMP-01-08   8   16  25  16.2    1.1  15.2
</pre>
<p>For RJ4JP-01-xx bearings (compatible with LMxxUU bearings):</p>
<pre>
// Parameters for RJ4JP-01-xx (shaft size xx, all dimensions in [mm])
// Designation  d1  d2  B   B1      s   dn
//-------------------------------------------
// RJ4JP-01-08  8   15  24  17.5    1.1  14.3
</pre>
<p><em>d1 is the inner diameter, d2 the outer diameter and B the length of the bearing</em></p>
<p>The RJ4JP-01-08 bearings are drop-in replacements for the LM8UU linear bearings. Note that the RJMP-01-08 have a larger outer diameter and do not fit into the regular bearing housings. The increased thickness gives the bearings more rigidity.</p>

CC BY-SA 4.0
2019-12-10T09:49:20.513Z
2019-12-11T05:36:45.587Z
2020-06-18T08:29:41.667Z
0scar
<p>I'm new to 3D printing and I'm learning how to build my own 3D printer.</p>

<p>I bought polymer LM8UU bearings and I'm trying to fit onto a rod of dimension: D8&nbsp;mm * L300&nbsp;mm. Sadly it doesn't fit, I was wondering if I would need to buy new rod? </p>

<p>If so, what size of diameter should be used?  General guidelines I need to look out for are also appreciated?</p>

CC BY-SA 4.0
2019-12-10T09:55:41.627Z
Welcome to 3D Printing.SE! Please explain if the bearing does not fit *into* some cavity or *onto* a shaft, that makes a big difference. As it is asked now it looks like the 2 are not compatible. My comment got too big to fit, so I posted an answer that also addresses fitting into something, hope this helps!
0scar
<diy-3d-printer><bearing>
Building 3D printer: LM8UU bearing can't fit into
Rookies DJ
# Building 3D printer: LM8UU bearing can't fit into

(Asked by: username_0 on 2019-12-10)

I'm new to 3D printing and I'm learning how to build my own 3D printer.

I bought polymer LM8UU bearings and I'm trying to fit onto a rod of dimension: D8 mm \* L300 mm. Sadly it doesn't fit, I was wondering if I would need to buy new rod?

If so, what size of diameter should be used? General guidelines I need to look out for are also appreciated?

**Tags:** diy-3d-printer, bearing
> **Comments:**
> **username_1**: Welcome to 3D Printing.SE! Please explain if the bearing does not fit *into* some cavity or *onto* a shaft, that makes a big difference. As it is asked now it looks like the 2 are not compatible. My comment got too big to fit, so I posted an answer that also addresses fitting into something, hope this helps!  

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## 1 answers

### Answer by username_1 on 2019-12-10. Score: 1 | Accepted answer

Bearing on a shaft
An 8 mm (ID, Internal Diameter) bearing should always fit on an 8 mm (OD, Outer Diameter) shaft. If it does not fit, one of the 2 is of a different size. Please use a caliper to determine the size of both.
Bearing in a housing
If it is the outer size of the bearing, i.e. fitting in a printed carriage or a bearing housing (e.g. SCxxUU), you should be aware of the different sizes of polymer bearings. E.g. there are RJMP-01-xx and RJ4JP-01-xx types of polymer bearings (xx denotes the shaft diameter, e.g. 08 for 8 mm shafts).
For RJMP-01-xx bearings (not compatible with LMxxUU bearings):

// Parameters for RJMP-01-xx (shaft size xx, all dimensions in [mm])
// Designation  d1  d2  B   B1      s   dn
//-------------------------------------------
// RJMP-01-08   8   16  25  16.2    1.1  15.2

For RJ4JP-01-xx bearings (compatible with LMxxUU bearings):

// Parameters for RJ4JP-01-xx (shaft size xx, all dimensions in [mm])
// Designation  d1  d2  B   B1      s   dn
//-------------------------------------------
// RJ4JP-01-08  8   15  24  17.5    1.1  14.3

d1 is the inner diameter, d2 the outer diameter and B the length of the bearing
The RJ4JP-01-08 bearings are drop-in replacements for the LM8UU linear bearings. Note that the RJMP-01-08 have a larger outer diameter and do not fit into the regular bearing housings. The increased thickness gives the bearings more rigidity.


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