Cast iron is the default housing material for pillow block bearings—and for good reason.
But "cast iron" isn't a single material.
There's HT150, HT200, HT250, HT300, and beyond.
The grade matters more than most buyers realize, and choosing the wrong one is one of the most common reasons pillow block housings crack during installation or fail prematurely in service.
This guide covers what makes cast iron work well as a bearing housing material, how to read the grade markings, when it outperforms other materials, and when you should switch to something else entirely.
A cast iron pillow block bearing is a pre-assembled mounted unit where the housing—the outer block that bolts to a mounting surface—is made from gray cast iron. Also referred to as pedestal bearings or plummer block bearings in some markets.
Inside sits a bearing insert, typically a deep-groove ball bearing or spherical roller bearing, along with seals and a locking mechanism to secure the assembly to the shaft.
The shaft runs parallel to the mounting surface.
Most standard cast iron pillow block housings follow the UCP or UKP series designation.
They're pre-greased, sealed, and ready to install straight from the box.
New to pillow block bearings? See our complete overview of types, components, and locking mechanisms:
What Are Pillow Block Bearings
What makes cast iron the dominant housing material isn't any single property—it's the combination of adequate strength, excellent vibration damping, good machinability, and low cost.
No other common housing material matches that combination at the same price point.
The "HT" prefix stands for gray cast iron.
The number that follows indicates the minimum tensile strength in MPa for a 30mm diameter test sample, per GB/T 9439-2010 standard (international equivalent: ISO 185:2019 — Grey Cast Irons Classification).
So HT250 means a minimum tensile strength of 250 MPa.
The grade gap that matters most
Many suppliers—particularly at the lower price end—use HT150 for pillow block housings.
At 150 MPa tensile strength, HT150 is already marginal.
But the real-world numbers are worse: at wall thicknesses of 30–40mm—typical for a pillow block housing boss—actual tensile strength drops to just 120 MPa.
A slightly overtightened housing bolt is all it takes to crack it.
This is one of the most common reasons cheap pillow block bearings fail during installation rather than in service—the housing gives out before the bearing does.
If a supplier can't confirm the grade, assume the worst and walk away.
Tensile strength: 200 MPa minimum | Compressive strength: ~620–700 MPa | Hardness: ~163–197 HB
The most common grade for standard duty cast iron pillow block bearings.
Adequate for general industrial conveyors, agricultural machinery, and light-to-medium shaft loads.
Cost-effective and widely available.
Tensile strength: 250 MPa minimum | Compressive strength: ~750 MPa | Hardness: ~180–230 HB (typically ~209 HB)
The preferred grade for most industrial applications.
Better wear resistance, higher load capacity, and more consistent dimensional stability than HT200.
Carbon content typically 3.16–3.30%, silicon 1.79–1.93%.
The European equivalent is EN-GJL-250; US equivalent is ASTM A48/A48M — Gray Iron Castings Grade 220 or Grade 260.
Tensile strength: 300 MPa minimum | Compressive strength: ~900 MPa | Hardness: ~187–255 HB (typically ~225 HB)
HT300 is the heavy-duty grade used in split housings, large-format pillow blocks, and high-load bearing applications across mining, paper mills, and power generation.
The higher strength comes from tighter control of the microstructure: carbon content typically runs 2.90–3.10%, silicon 1.40–1.70%—lower than HT250, which produces finer pearlite and fewer free ferrite zones. The result is higher hardness, better wear resistance, and significantly improved fatigue resistance under cyclic loading.
The European equivalent is EN-GJL-300; the closest US equivalent is ASTM A48 Grade 300.
Manufacturing cost is higher than HT250—expect a 15–25% price premium on the housing—but for applications with continuous heavy radial loads or cyclic stress (paper mill rolls, crusher drive shafts, large fan housings), the fatigue life improvement justifies the cost difference.
When to specify HT300 over HT250:
For standard industrial use—conveyors, pumps, gearboxes, agricultural drives—HT250 is sufficient and more cost-effective. HT300 is not a universal upgrade; it's a targeted choice for genuinely demanding conditions.
Quick rule of thumb
For standard industrial use, specify HT250 as a minimum. If a supplier only offers HT200, it's still acceptable for light duty. Reserve HT300 for heavy cyclic loads, large shaft diameters, or 24/7 high-load operations. If they can't confirm the grade at all, look elsewhere.
The standard tensile strength figure (e.g. "250 MPa" for HT250) applies to a single 30mm test specimen. Actual strength in a real casting varies with wall thickness—thicker sections cool more slowly, producing coarser graphite and lower strength. This is why housing wall thickness matters when specifying a grade.
|
Gray Iron Grade |
Single Specimen Tensile Strength (MPa) |
Wall Thickness (mm) |
Actual Casting Tensile Strength (MPa) |
|
HT100
|
100
|
>2.5–10 |
130 |
|
>10–20 |
100 |
||
|
>20–30 |
90 |
||
|
>30–40 |
80 |
||
|
HT150
|
150
|
>2.5–10 |
175 |
|
>10–20 |
145 |
||
|
>20–30 |
130 |
||
|
>30–40 |
120 ⚠️ — inadequate for most housings |
||
|
HT200
|
200
|
>2.5–10 |
220 |
|
>10–20 |
195 |
||
|
>20–30 |
170 |
||
|
>30–40 |
160 |
||
|
HT250
|
250
|
>4.0–10 |
270 |
|
>10–20 |
240 |
||
|
>20–30 |
220 |
||
|
>30–50 |
200 |
||
|
HT300
|
300
|
>10–20 |
290 |
|
>20–30 |
250 |
||
|
>30–50 |
230 |
||
|
HT350
|
350
|
>10–20 |
340 |
|
>20–30 |
290 |
Source: GB/T 9439-2010, Gray Cast Iron Standard. HT150 at 30–40mm wall thickness drops to 120 MPa—inadequate for most pillow block housing applications.
✅ Quick rule of thumb
For standard industrial use, specify HT250 as a minimum. If a supplier only offers HT200, it's still acceptable for light duty. Reserve HT300 for heavy cyclic loads, large shaft diameters, or 24/7 high-load operations. If they can't confirm the grade at all, look elsewhere.
Gray cast iron contains interconnected flake graphite distributed throughout its metal matrix.
These graphite flakes interrupt the propagation of stress waves and convert vibrational energy into heat through internal friction—a mechanism called internal damping.
The numbers: gray cast iron has roughly twice the damping capacity of ductile iron, and ductile iron has roughly twice that of steel (U.S. Bureau of Mines, RI-9068).
In practical terms, a cast iron pillow block housing running on a vibrating conveyor or pump protects the bearing insert significantly better than a steel housing under the same conditions.
This is exactly why gray cast iron—despite its relatively modest tensile strength—remains the dominant material for machine tool beds, engine blocks, and bearing housings worldwide.
Gray cast iron's tensile strength is modest—200–300 MPa depending on grade.
But its compressive strength is 3 to 4 times higher than its tensile strength.
HT200 reaches around 600 MPa in compression; HT250 around 750 MPa; HT300 around 900 MPa.
For a pillow block housing that's primarily loaded in compression by the shaft and bolts, this asymmetry works in cast iron's favor.
Cast iron has a thermal conductivity of approximately 46–50 W/(m·K)—roughly 3 times higher than thermoplastics.
In a bearing housing, this means heat generated by the rotating bearing insert dissipates into the housing and surrounding structure rather than concentrating at the contact zone.
Bearings running cooler last longer, all else being equal.
Gray cast iron machines well.
The graphite flakes act as a built-in lubricant, producing short, brittle chips rather than the long stringy chips you get with steel.
This makes it easier and cheaper to machine the housing bore to tight tolerances, which directly affects bearing fit and performance.
Lily Bearing maintains hole-to-hole tolerances within ±0.2mm across our cast iron housing range.
When exposed to air and moisture, gray cast iron forms a passive iron oxide layer that provides basic corrosion protection.
It's not as robust as the passive layer on stainless steel, but it does provide meaningful protection in dry indoor environments and moderate outdoor exposure.
Surface coatings—paint, black oxide, powder coat—extend this protection further.
* 250°C refers to the cast iron housing material limit. The bearing insert and grease typically impose a lower practical limit—standard NLGI #2 grease begins to degrade above 120°C, and the bearing insert may require specialty lubrication above that threshold.
Ductile iron (also called nodular or spheroidal graphite iron) has graphite in spherical form rather than flakes.
That change in graphite morphology dramatically improves tensile strength and impact resistance—but it also eliminates the interconnected graphite network that makes gray iron such an effective vibration damper.
If your application involves heavy shock loads or high tensile stress, ductile iron is the better choice.
If vibration damping is the priority, gray cast iron wins.
Most buyers who specify cast steel housings for standard conveyor or pump applications are over-engineering the solution.
Cast steel's tensile strength of 480–620 MPa and excellent impact resistance are genuinely needed for extreme applications—jaw crushers, hammermills, heavy press equipment.
For everything else, you're paying a significant cost premium and giving up vibration damping in exchange for strength you'll never use.
Gray cast iron's compressive strength of ~750 MPa (HT250) is more than adequate for the compression-dominant loading that pillow block housings actually experience.
Stainless steel (304 or 316 grade) has tensile strength above 515 MPa and excellent corrosion resistance in wet, chemical, or food-grade environments.
It costs significantly more than cast iron.
For dry industrial environments with no special hygiene requirements, cast iron makes more economic sense.
For anything involving frequent washdown, moisture exposure, or food contact, stainless steel is the correct choice regardless of cost.
Application involves vibration or noise reduction (conveyors, pumps, fans)
Loads are radial and moderate to heavy, but not severe impact
Operating temperature stays below 120°C (grease and insert limit; housing itself tolerates up to ~250°C)
Environment is dry or moderately humid indoors
Budget is a primary consideration
Standard industrial machinery: gearboxes, agricultural drives, HVAC equipment
Frequent washdown or submersion (use stainless steel)
Food, pharmaceutical, or FDA-regulated environments (use stainless steel or thermoplastic)
Severe shock or impact loads—rock crushers, hammermills (use ductile iron or cast steel)
Corrosive chemical exposure (use stainless steel or thermoplastic)
Marine or saltwater environments (use stainless steel with appropriate seals)
Operating temperature above 250°C sustained (use cast steel or ductile iron)
Before selecting a housing material, confirm you have the right bore size for your shaft: Pillow Block Bearing Size Chart
ℹ️ A note on "food processing" applications
Cast iron is sometimes used in dry food processing (grain handling, feed mills) where washdown isn't frequent and contamination risk is low.
But for any wet food processing—meat, dairy, beverage—stainless steel or food-grade thermoplastic is the only appropriate choice.
Don't compromise on this.
Deciding between a pillow block and a flange unit for your application?
Pillow Block vs. Flange Units: Full Comparison →
Cast iron pillow block bearings are available in a wide range of bore sizes to match standard shaft diameters. Most housings follow the UCP (set screw locking) or UKP (adapter sleeve) series in metric sizing, with inch-bore variants widely used in North American and agricultural applications.
The most commonly ordered inch bore sizes are 3/4″ (19.05mm) and 1″ (25.4mm) in the UCP 200 series, and 1-1/2″ (38.1mm) and 1-3/4″ (44.45mm) in the UCP 300 series. These cover the majority of agricultural, conveyor, and general industrial shaft sizes.
When ordering, specify bore size, series, and locking mechanism (set screw vs. adapter sleeve / UKP series) to ensure the correct fit. A housing selected for the wrong bore will affect bearing preload, alignment, and ultimately service life.
For a full size reference including shaft tolerances and housing dimensions: Pillow Block Bearing Size Chart
Cast iron pillow blocks are the workhorses of conveyor drives.
They support the drive shaft and idler rollers, absorb the vibration from uneven belt loading, and handle the continuous radial loads without the need for frequent maintenance.
The vibration damping reduces noise in enclosed conveyor structures—a practical benefit in manufacturing plants where noise regulations apply.
Harvesters, balers, seeders, and grain elevator conveyors all run in environments with heavy dust, moisture, fertilizer exposure, and shock loads from debris.
Cast iron's natural oxide layer provides baseline corrosion protection, and its compressive strength handles the heavy radial loads typical of agricultural drives.
Triple-lip seals paired with cast iron housings extend service life significantly in these conditions.
Seal selection matters as much as housing material in agricultural and dusty environments: Pillow Block Bearing Seals: Lip vs. Labyrinth Guide
Fans and pumps generate continuous vibration from rotating imbalance and fluid dynamics.
Cast iron housings absorb this vibration and prevent it from transmitting into the shaft and bearing insert—reducing wear on internal components and extending service life.
The thermal conductivity also helps manage heat buildup in continuously running equipment.
Gear meshing generates both radial load and significant vibration at gear mesh frequency.
Cast iron housings dampen this vibration, protecting adjacent bearings and reducing audible noise.
This is why gray cast iron remains the dominant material for gearbox housings despite decades of alternative materials being available.
Concrete mixers, rock crushers, and earth-moving equipment expose bearings to abrasive dust and shock loads.
Cast iron's compressive strength handles the loads; paired with labyrinth or taconite-style seals, the housing keeps abrasive contamination away from the bearing insert.
For the most severe shock applications—jaw crushers, for example—ductile iron or cast steel housings are a better fit.
Cast iron housings themselves need very little attention—they don't wear, they don't corrode quickly, and they don't need adjustment.
The maintenance focus is on the bearing insert inside, not the housing.
Keep grease fresh.
Most cast iron pillow blocks ship pre-packed with NLGI #2 lithium complex grease.
In clean indoor environments, re-lubrication intervals run 1,500–2,500 hours. In agricultural or dusty settings, that drops to around 200 hours.
Always use a grease compatible with the factory fill.
Use Class 8.8 bolts minimum and torque to spec.
Cast iron doesn't yield like steel—overtightening cracks the housing boss rather than stretching the bolt.
Check housings quarterly for hairline cracks around the bolt bosses, and watch for abnormal noise, excessive vibration, or grease leaking at the seals.
Any of these signals a problem with the bearing insert, not the housing itself.
For grease selection, re-lubrication intervals, and full failure diagnosis: Best Grease for Pillow Block Bearings
For failure causes and prevention: Common Failures and How to Avoid Them
Most quality pillow block bearing housings use HT250 or HT300 gray cast iron, offering tensile strengths of 250 MPa and 300 MPa respectively.
HT200 (200 MPa) is an acceptable mid-range option.
Avoid HT150—it's too brittle for most industrial applications and prone to cracking during installation or under load.
If a supplier can't confirm the grade, that's a red flag.
Gray cast iron contains interconnected flake graphite distributed throughout its matrix.
These graphite flakes interrupt stress wave propagation and convert vibrational energy into heat through internal friction.
Gray cast iron has roughly twice the damping capacity of ductile iron, and ductile iron has roughly twice that of steel.
This is why gray cast iron remains the standard material for pillow block bearing housings and machine tool beds in vibration-prone applications.
Avoid cast iron in: frequent washdown or submersion environments (use stainless steel), food processing or pharmaceutical applications requiring FDA compliance (use stainless steel or thermoplastic), severe shock load applications like jaw crushers or hammermills (use ductile iron or cast steel), and corrosive chemical exposure environments.
For everything else—general industrial, agricultural, HVAC, conveyor—cast iron is usually the right call.
HT200 has a minimum tensile strength of 200 MPa and compressive strength of 620–700 MPa.
HT250 has a minimum tensile strength of 250 MPa, compressive strength around 750 MPa, and hardness of approximately 209 HB.
For standard applications, HT200 works. For heavier loads, higher speeds, or significant vibration, HT250 is the better choice and worth specifying explicitly when ordering.
HT300 gray cast iron has a minimum tensile strength of 300 MPa and compressive strength of approximately 900 MPa, with typical hardness of 187–255 HB. Carbon content runs 2.90–3.10% and silicon 1.40–1.70%—tighter than HT250, producing finer pearlite and improved fatigue resistance. The European equivalent is EN-GJL-300; the US equivalent is ASTM A48 Grade 300. It is used in heavy-duty pillow block applications including mining equipment, paper mill rolls, and large-format power transmission housings where cyclic loading or large shaft diameters demand higher material performance than HT250 provides.
Cast iron pillow block bearings in the UCP 200 series cover bore sizes from 12mm to 60mm, with common inch sizes including 1/2″, 3/4″, 1″, and 1-1/2″. The UCP 300 series extends this range up to 90mm, covering shaft sizes up to 3-7/16″. Always match the bore to your exact shaft diameter and specify the locking method—set screw or adapter sleeve—when ordering.
Yes, with limitations. Cast iron forms a natural oxide layer that provides basic corrosion protection outdoors.
A surface coating—paint, black oxide, or powder coat—extends this significantly.
For dry outdoor applications like agricultural equipment or construction machinery, cast iron with appropriate seals performs well.
For marine environments, saltwater exposure, or anything involving regular water contact, switch to stainless steel housings.
Cast iron became the default pillow block housing material for a reason.
The vibration damping alone justifies it in most industrial applications, and the combination of strength, machinability, and cost is hard to beat.
But "cast iron" isn't one thing—HT250 and HT150 are worlds apart in practice, and knowing which grade you're getting matters.
If your application fits the profile—moderate-to-heavy radial loads, vibration-prone environment, temperatures below 120°C operating, no special hygiene requirements—cast iron is the right choice.
If it doesn't fit, now you know what to switch to and why.
Ready to install? Our step-by-step guide covers shaft prep, housing alignment, torque values, and expansion unit setup: How to Install Pillow Block Bearings →
Technical note:
Material property data references GB/T 9439-2010 (gray cast iron standard), with international equivalents EN-GJL-250, EN-GJL-300, and ASTM A48. Vibration damping comparisons reference Visnapuu, Nash & Turner (1987), U.S. Bureau of Mines Report RI-9068, "Damping Properties of Selected Steels and Cast Irons." Always verify cast iron pillow block housing grade specifications with your supplier before ordering for critical applications. Lily Bearing assumes no liability for applications specified without consulting the relevant manufacturer documentation.
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