The Decision That Repeats in Every Mechanism

Anything that rotates or slides needs a bearing, and most product teams treat it as a catalog lookup at the end of the design. That is backwards. The bearing sets the shaft diameter, the housing bore tolerance, the seal strategy, the assembly method, and often the failure mode that shows up in month eight of field service. Picking it early costs nothing. Picking it late costs a redesign. Here is how to choose among the three families you will realistically consider, size the choice against load and life, and specify it so the shop can build it.

The Three Families and What Each One Is For

Plain bushings. A sleeve of bronze, sintered oil-impregnated bronze, filled PTFE, or engineering plastic. No rolling elements, nothing to contaminate. Cheapest by far at $0.20 to $3 in volume, quietest, most tolerant of shock and dirt, and the smallest radial envelope for a given shaft. They give up efficiency: friction coefficient is typically 0.05 to 0.20, versus roughly 0.001 to 0.005 for a ball bearing. They also wear steadily rather than failing suddenly, which is often the friendlier behavior in a consumer product.

Deep-groove ball bearings. The default when speed matters, when starting torque must be low and predictable, or when the shaft must stay precisely located. A 608 or 625 shielded bearing costs $0.50 to $4 depending on quality tier and quantity, and handles combined radial and modest axial load. They are far less tolerant of misalignment than a bushing: 0.1 degrees of shaft angular error shortens life substantially, so housing bore and shaft concentricity matter as much as the bearing.

Needle and cylindrical roller bearings. Line contact instead of point contact, so radial capacity per unit of envelope is much higher. Use them when load is high and space is tight. They carry essentially no axial load and need a hardened, ground raceway on the shaft or the housing, so they cost more to design in even when the part is cheap.

A Fast Selection Path

  • Speed below roughly 100 rpm, load moderate, cost sensitive: plain bushing. Most consumer product pivots, hinges, and slow rollers belong here.
  • Continuous rotation, any meaningful speed, efficiency or torque ripple matters: ball bearing. Anything driven by a motor almost always ends up here, and the motor sizing math in how to calculate motor torque depends on which one you chose.
  • High radial load in a small radial envelope: needle bearing, with a hardened counterface.
  • Axial load is the dominant load: thrust washer or thrust bearing, added separately. Deep-groove ball bearings take only a fraction of their radial rating axially.
  • Dirty, wet, or unlubricated by design: filled PTFE or plastic bushing, and accept the wear rate.

Load and Life, Without the Handbook

Rolling bearing catalogs give a dynamic load rating C. Rated life in millions of revolutions is (C divided by the applied load P) raised to the third power for ball bearings, and to the 10/3 power for rollers. The exponent is the important part: halving the load multiplies life by eight. If your calculated life is marginal, do not shop for a better bearing first. Reduce the load, shorten the moment arm, or add a second support point.

Convert the answer into hours at your actual duty cycle, then compare it to the warranty period plus a margin. A product running four hours a day for five years needs about 7,300 operating hours, and the calculation should land well above that, not at it. How much margin is enough is the judgment call in factor of safety in mechanical design.

For plain bushings the governing number is PV, pressure times surface velocity. Suppliers publish a PV limit plus a maximum P and a maximum V, and all three must be satisfied independently. Exceeding V alone melts a plastic bushing at trivial load.

The Detail That Kills More Bearings Than the Bearing Choice: The Fit

A bearing pressed on both the inner and outer race has nowhere to go when the shaft grows thermally, and it will preload itself to death. Standard practice is one interference fit and one slip fit: interference on the rotating race, slip on the stationary one, with the outer race free to float axially in the housing. For a ball bearing on a rotating shaft that is roughly a k5 or m5 shaft and an H7 housing bore. Getting those symbols right is the subject of GD&T basics, and squeezing the outer race with an oversized press fit reduces internal clearance, raises running temperature, and turns a 20,000 hour bearing into a 2,000 hour one.

Also specify surface finish and shoulder squareness. A bearing seated against an out-of-square shoulder is a misaligned bearing regardless of how good the diameters are.

Sealing, Lubrication, and Environment

Shielded bearings (ZZ) keep out large debris and add almost no drag. Sealed bearings (2RS) keep out water and fine dust but add measurable friction, which matters in a battery-powered product. Neither substitutes for a proper enclosure seal if the product sees washdown or rain, which is the separate problem in O-ring selection and gland design. Note too that grease has a service life of its own, usually shorter than the bearing's fatigue life in a sealed-for-life unit.

Reading a Failed Bearing

A bearing that comes back from the field is a diagnostic report. Even wear around the rotating race is normal life. Wear on only one side of the raceway means misalignment. Brinelling, small evenly spaced dents, means shock load or press-fit damage during assembly, usually from pressing on the wrong race. Discoloration means overheating from excess preload or lost lubricant. Fine reddish paste means fretting against a loose housing. Each points to a different fix, and only one of them is "buy a better bearing."

Sourcing and Cost Reality

Stick to standard metric sizes. A 608, 625, 6800 series, or 6000 series bearing is available from a dozen sources within days. A custom bore diameter is a six-week lead time and a single-source risk. This is the classic case for the tradeoff in off-the-shelf versus custom components: design the shaft to the bearing, not the bearing to the shaft.

Quality tier matters more than founders expect. An unbranded bearing at a third of the price often has a different internal clearance class and unspecified grease. In a hand tool nobody notices; in a quiet appliance it is the difference between a product that hums and one that whines.

Get the Bearings Right Before the Housing Is Tooled

Projects House sizes bearings, shafts, and housing fits as part of mechanical design, including the life calculation and the drawing callouts your machinist needs. Describe the mechanism, the loads, and the duty cycle through our contact form and we will work through the selection with you.