What are loose ball bearings?
Loose ball bearings are individual precision balls supplied on their own rather than assembled into a complete bearing. They are also known as bearing balls, precision balls or simply steel balls. In a complete ball bearing, the balls are the rolling elements that sit between the inner and outer rings. When sold loose, the same balls are used to build, service or repair bearings and other mechanisms. They are manufactured to very tight tolerances for size, roundness, and surface finish. Accuracy is classified by grade.
They are made in a wide range of materials, including:
- Chrome steel
- Stainless steel
- Carbon steel
- Ceramics
- Tungsten carbide
- Brass
- Glass
- Plastics
Sizes range from fractions of a millimetre to several centimetres, in both metric and imperial dimensions.
Why are loose ball bearings essential?
Many bearing designs don’t use a sealed, self-contained cartridge. Instead, they rely on loose balls running directly between cups and cones or other raceways, so replacement balls are needed to service them. They are a key part of maintaining traditional bicycle bearings, including cup-and-cone hubs, threaded headsets, older bottom brackets and some pedals. Replacing worn or pitted balls can restore smooth running cheaply.
Their uses go well beyond bearings, including:
- Check valves and pump valves, where a ball seals against a seat to allow flow in one direction only
- Ball transfer units on conveyor tables and handling equipment
- Detent and locking mechanisms, spring plungers and indexing devices
- Drawer slides, linear guides and ball screws
- Swivel and turntable mechanisms
- Ballpoint pen tips
- Mixing balls in aerosol spray paint cans
- Gauging, measurement and calibration work
Because a ball is a simple, accurately made shape, it is also widely used in engineering, prototyping, model making and education. Using the correct size, grade, and material helps ensure smooth operation, accurate sealing in valves, low wear, and a long service life.
The main types of loose ball bearings
- Chrome steel balls - are the most common type; they are made from high-carbon chromium bearing steel (AISI 52100) and are through-hardened, typically to around 60-66 HRC. They offer high hardness, good wear resistance, and high load capacity at a low cost. They are magnetic and will rust if exposed to moisture without protection, so they need lubrication or a dry environment. Typical uses include bearings, bicycle hubs and headsets, ball screws, and general engineering.
- Martensitic stainless steel balls (AISI 440C, sometimes 420) - 440C balls are typically hardened to around 58-65 HRC. They combine good hardness with better corrosion resistance than chrome steel, though they are not fully rust-proof in harsh conditions. They are magnetic, and typical uses include bearings in damp conditions, food and medical equipment, and valves.
- Austenitic stainless steel balls (AISI 302, 304 and 316) - are softer but more corrosion-resistant. They cannot be hardened by heat treatment, so they are not suited to heavily loaded bearings. They offer very good corrosion resistance, especially grade 316 in chemical and marine environments. They are largely non-magnetic, although cold working during manufacture can make them slightly magnetic. Typical uses include valves, sprayers, aerosol cans, and food and chemical equipment.
- Carbon steel balls - these are a low-cost option and are usually case-hardened; this gives them a hard surface over a softer core. They suit light-duty applications such as drawer slides, castors, toys and low-precision mechanisms. They rust easily unless coated or kept oiled.
- Ceramic balls - they are made from silicon nitride, zirconia or alumina. Silicon nitride balls are much lighter than steel, very hard, and electrically insulating. They are widely used in hybrid bearings for high speeds and electric motors. Zirconia and alumina balls offer high corrosion resistance and are non-magnetic. Typical uses include high-speed bearings, chemical pumps and valves, vacuum and non-magnetic applications, and high-performance bicycle bearings.
- Tungsten carbide balls - these are extremely hard, dense, and wear-resistant. Typical uses include valve seats, gauging, ballpoint pen tips, and high-wear applications.
- Brass balls - these are non-magnetic, resist corrosion in many environments, and don’t spark. Typical uses include valves, electrical applications, and decorative items.
- Glass balls - These are chemically resistant and non-conductive. Typical uses include valves, flow meters, sprayers and laboratory equipment.
- Plastic balls - Plastic balls are lightweight, quiet, corrosion-resistant, and non-conductive and are made from materials such as acetal (POM), nylon, polypropylene, and PTFE. These balls only suit light loads and typical uses include valves, light-duty bearings and slides, and fast food or chemical equipment.
How to choose the right loose ball bearings?
- Size - Match the diameter exactly to the original balls or the manufacturer’s specification. Even small differences can cause rough running or uneven loading. Be careful not to confuse metric and imperial sizes, as some are very close. Measure with a micrometer or callipers if the size is not known, and always measure several balls.
- Grade (precision) - Grades under ISO 3290 include G3, G5, G10, G16, G20, G28, G40, G100, G200, G500, G1000. The lower the grade number, the more precise the ball, with tighter limits on sphericity, surface roughness, and size variation within a lot. For example, a G10 ball has a maximum deviation from spherical form of 0.25 µm, while a G100 ball allows 2.5 µm. High grades (G3 to G16) suit precision bearings, instruments and gauging. Mid grades (G20 to G40) suit good-quality bearings, including most bicycle bearings. Lower grades (G100 and above) suit valves, slides, castors and general mechanisms.
- Material - Use chrome steel for strength, hardness, and value in lubricated, dry conditions. Use 440C stainless for corrosion resistance combined with hardness. Use 316 stainless, glass, or plastic for chemical or wet environments with light loads. Use ceramic for high speeds, low weight, electrical insulation, or non-magnetic needs.
- Load and hardness - Bearings and other loaded applications need through-hardened balls, such as chrome steel, 440C, or ceramic. Softer balls, such as austenitic stainless, brass, or plastic, will deform under heavy loads.
- Matching the raceways - In bearings, the balls should be as hard as or slightly harder than the raceways. Very hard ceramic balls running on worn or soft steel raceways can accelerate wear on the raceways.
- Quantity - Always replace every ball in a bearing at the same time, using balls of the same size, grade, and material, ideally from one batch. Mixing old and new balls, or balls of different sizes, means a few balls carry most of the load, which causes rough running and rapid wear.
- Magnetic properties - Choose ceramic, brass, glass, plastic or austenitic stainless balls where magnetism must be avoided.
Essential references and specialised ranges
Common imperial sizes
Common metric sizes
1mm, 1.5mm, 2mm, 2.5mm, 3mm, 4mm, 5mm, 6mm, 8mm, 10mm, 12mm, and many intermediate and larger sizes also available.
Typical bicycle sizes
- Front hubs commonly use 3/16” balls
- Rear hubs commonly use 1/4” balls
- Traditional bottom brackets commonly use 1/4” balls
- Headsets commonly use 5/32” or 3/16” balls, often in retainers
- Pedals commonly use 5/32” or 1/8” balls
FAQ - All you need to know about loose ball bearings
What is the difference between loose balls and a sealed bearing?
Loose balls are only the rolling elements. They run directly on raceways built into the component, such as the cups and cones of a bicycle hub. A sealed bearing is a complete self-contained unit with its own rings, balls, cage, and seals, and is replaced as a whole.
How do I measure a loose ball?
Use a micrometer for the best accuracy, or digital callipers for general identification. Measure several balls and compare them with standard metric and imperial size charts.
Can I mix balls from different batches or grades?
It is best not to. Even small size differences mean the larger balls carry more of the load, which causes rough running and faster wear. Use a full set from one pack whenever possible.
Can I reuse old loose ball bearings?
It is generally not recommended. Worn balls may be slightly undersized, flattened, or pitted, even if they look fine. New balls are inexpensive, so most mechanics replace them whenever a bearing is serviced.
