Pump Bearing Fits confusion

[Daren Samy]

In vertical multi stage pump drawing, guide bearing has fit 120H8/f9. When I calculate it is clearance fit with
Min clearance: .036mm
Max clearance: 0.177mm

Where as in Pump instruction book, nomianal clearances for guide bearing are as under
Min clearance : 0.15mm
Max clearance: 0.4mm

Kindly educate me whether my calculations are correct and if yes, then why two values are different?
If my calculations are wrong then what is mistake?

 

[Burunduk]

For what it's worth, I looked at ANSI B4.2-1978 Preferred Metric Limits and fits and got the same results as you did.
120H8 is 120 +0.054/0.00 (table A5, hole)
120f9 is 120 -0.036/-0.123 (table A15, shaft).

Therefore, what you wrote seems correct:
"Min clearance: .036mm
Max clearance: 0.177mm"

 

[Daren Samy]

Okay. But why guide bearing clearance data provided in instruction book by same vendor is different than drawing ? Is it conservative approach ? or thermodynamic conductions create the difference ?
According to drawing clearance should be between .036 to 0.177mm . If this value is diammetrical then then it would be .018 to .88mm in radial terms.
Where as in instruction book , it is 0.15mm to 0.4mm. Why such a difference ?

Daren!

 

[Burunduk]

If the source of both documentations (instruction book and pump drawing) is the same vendor, I think the best thing you can do is contact that vendor and ask why there is an exception/discrepancy.

 

[Daren Samy]

These are rubber bearings installed in vertical condensate pumps. This is not only in case of bearings. Wear ring clearance and sleeve clearance are not exactly the same as mentioned in drawings. Yes! Both documents are from same vendor.

Daren!

 

[geesaman.d]

Consider the possibility that the pump guide bearing clearance range was in practice before it was converted to ISO fits. So the conversion could be approximate and conservative.

I'm lazy and I use this for ISO fits:

https://amesweb.info/fits-tolerances/tolerance-calculator.aspx

 

[Daren Samy]

Geesaman ! May be right

Daren!