Grey iron, spheroidal iron or cast steel — the housing material
The same housing in a stronger material takes roughly 1,8 times more load and changes not a single mounting dimension. It is the cheapest strength reserve you can buy.
Four designations you will meet
The material sits in the suffix of the housing designation — „SNI 520 GGG” is the same housing as „SNI 520”, only in spheroidal iron.
| Designation | What it is | Where used |
|---|---|---|
| GG 20 | grey cast iron | standard in SNI and SN housings |
| GG 25 | higher-grade grey cast iron | standard in SD housings |
| GGG 40 | spheroidal cast iron | higher loads, impact duty |
| GS 45 | cast steel | highest loads |
What the stronger material buys
The catalogue states it plainly: for housings in GGG 40 and GS 45 the breaking load is multiplied by a factor of about 1,8 over grey iron.
The dimensions do not change. Same housing, same bolt spacing, same shaft centre height — only the casting differs. Mass changes, because there is more material.
1,8× the breaking load without changing a single mounting dimension.
When it pays off
Under impact loading. Grey iron is brittle — it cracks without warning. Spheroidal iron has a defined impact energy even at minus 20 °C and clearly higher elongation at break, so under a blow it deforms instead of cracking.
When the force acts at an angle. The catalogue rates housings for loads perpendicular to the split plane; at other angles the breaking load drops and the stronger material covers the difference.
When there is no room for a bigger housing. Instead of redesigning the foundation for a larger size, take the same size in GGG 40.
When it makes no sense
When the bearing is the limit, not the housing. The housing breaking load is the point at which the iron fails — if the bearing gives out first, a stronger casting changes nothing.
When the bolts are the limit. The bolts joining the two halves are class 8.8 and have their own permissible loads. With an angled force they, not the material, often set the limit.
Before paying for material, check the bearing rating and the bolt class. The weakest link is rarely where you look for it.
Cast steel instead of spheroidal iron
The manufacturer offers spheroidal iron as a frequent substitute for cast steel. The difference in breaking load is small — both groups get the same 1,8 factor — and a spheroidal casting is cheaper and more readily available.
GS 45 cast steel remains where the customer's specification or an industry standard requires it.
Source
HFB catalogue 2011/03, pages 6 and 34 (materials, 1,8× factor) and the manufacturer's SDI leaflet.