A mounted unit is the simplest answer to a common problem: how do you support a shaft with a bearing that is easy to fit, easy to align and easy to replace in the field? The answer is to combine two parts - a self-aligning insert bearing and a housing that carries it - into one bolt-on assembly. That is why the same bearing can be supplied as a pillow block, a two-bolt flange, a four-bolt flange or a take-up unit.
This guide decodes the designations in daily use - UCP, UCF, UCFL, UCT, UCFC, UCPA and the UK variants - explains the insert bearing inside them, compares the locking methods and housing materials, and walks through a mounting sequence that avoids the mistakes which cause most premature failures in conveyors, fans and farm machinery.
- A mounted unit is an insert plus a housing. The insert is a self-aligning bearing with a spherical outside diameter; the housing is a cast or pressed shape that bolts to the machine frame.
- The letter after UC tells you the housing. P is a pillow block, F a four-bolt flange, FL a two-bolt flange, T a take-up, FC a flanged cartridge and PA a two-bolt pillow block.
- The trailing digits are the bore in millimetres. A UCP 205 takes a 25 mm shaft, and the same 205 bore appears in every housing shape.
- Locking method sets the duty. Set screws suit light duty, an eccentric collar suits reversing and vibration, and an adapter sleeve suits heavier loads and larger shafts.
What a mounted unit actually is
Two parts, one assembly. The insert bearing - often called a Y-bearing or UC bearing - is a single-row ball bearing with a wide inner ring, a spherical outside diameter on the outer ring and a locking device on the inner ring. That spherical OD is the key feature: it lets the bearing tilt inside the housing to follow small shaft misalignment and deflection, which is exactly what a conveyor, a fan or a drive shaft does under load.
The housing holds the insert, provides the mounting bolt holes and protects the bearing from dust and water. Because the insert is replaceable, the whole unit does not need to be discarded when the bearing wears. Our ball bearing unit range lists the standard sizes in every housing shape, and the insert itself belongs to the self-aligning ball family in terms of its internal design.
Decoding UCP, UCF, UCFL, UCT, UCFC, UCPA and UKP
The housing letter
The prefix fixes the housing shape, and the shape fixes the footprint. Getting the letter right before ordering is more important than the bore, because two units with the same bore and different letters cannot be swapped without redrilling the frame.
The bore code and the metric versus inch convention
The trailing digits are the bore in millimetres for the metric series, so a 205 unit takes a 25 mm shaft, a 206 a 30 mm shaft and a 207 a 35 mm shaft. The same bore is also produced in inch sizes under a different numbering, and the two are not interchangeable on the shaft - always state millimetre or inch explicitly and measure the shaft rather than trusting a drawing callout. A 206 takes a 30 mm shaft, a 207 a 35 mm shaft and a 208 a 40 mm shaft.
| Housing letter | Shape | Mounting | Typical position |
|---|---|---|---|
| P | Pillow block | Two bolts, base mount | Shaft runs above the frame |
| PA | Two-bolt pillow block, lighter | Two bolts, base mount | Light duty, tight space |
| F | Four-bolt flange | Four bolts, face mount | Frame face perpendicular to shaft |
| FL | Two-bolt flange | Two bolts, face mount | Compact face-mount positions |
| T | Take-up | Slides in a frame slot | Belt tensioning and adjustment |
| FC | Flanged cartridge | Circular, locked by set screws | Bore-mounted in a tube or plate |
| UKP | Pillow block with adapter sleeve | Two bolts, base mount | Heavier duty on plain shafts |
Insert bearings: UC, UE, SA and the Y-bearing
The insert is where the engineering happens. A UC insert has a wide inner ring, a spherical outer ring OD and a refined seal arrangement; the same envelope can be had with a set screw lock (UC), an eccentric collar (UE), a plain wide inner ring (SA and SB) or an adapter-sleeve lock (UK). The bore-to-outside relationship follows ISO 9628 for the metric insert series, the housed unit dimensions follow ISO 3228 and the insert boundary dimensions follow ISO 15, which is why an insert from one maker drops into another maker's housing of the same size.
The wide inner ring gives the locking device its real estate, and it also gives the bearing a longer contact area on the shaft, which reduces shaft wear at the lock. That is the reason a mounted unit tolerates the marginal shaft finish and misalignment common in field installations far better than a precision radial bearing would.
Two-bolt and four-bolt flange layouts
The flange units mount to a face rather than a base, and the bolt pattern is the whole decision. A two-bolt flange puts the bolts on a horizontal centre line and reaches into positions where a four-bolt flange is too wide; a four-bolt flange spreads the load over a square pattern and resists rocking better, which matters on a frame that flexes.
Beware the tolerance stack on a flange face. Both bolt holes and the spigot must be square to the shaft axis, and a flange bolted to a face that is out of square loads the insert at an angle. Because the insert self-aligns, the bearing survives a few tenths of a degree; beyond that the spherical seat is held at its travel limit and the alignment error passes straight into the balls. Aim for the spigot and the bolt face to be square within 0.1 mm across the mounting face, and remember that the insert itself self-aligns by only about 2°. The pillow block selection guide covers the load side of the same choice.
Locking methods and their duty trade-offs
Set screw
Two set screws in the inner ring bear on the shaft. It is the quickest and cheapest lock, and it is fine for light duty at moderate speed. Tighten the two screws to roughly 4 to 8 N·m on a 25 mm shaft; the limit is the shaft: a set screw on a hardened shaft cannot bite, and it creeps under vibration, so it is a poor choice for a reversing drive or a high-vibration conveyor.
Eccentric collar
An eccentric collar wraps the inner ring extension and locks when it is turned against the shaft. It distributes the grip over a larger area than a set screw, handles reversing and vibration better and is far less likely to score the shaft. The trade is that the collar must be turned in the direction of shaft rotation and then tightened firmly - an over-tightened collar distorts the inner ring and pinches the balls. On a 205 unit the collar is typically tightened to about 15 to 20 N·m.
Adapter sleeve
The UK series uses a tapered adapter sleeve with a lock nut and washer. It is the choice for larger shafts and heavier loads, because the grip is generated by the taper rather than by a point contact, and it allows the bearing to be positioned accurately along the shaft. The larger sizes, from UK 210 on a 50 mm shaft upward, are where the sleeve becomes the standard choice. The sleeve system also reuses the lock nut and adapter components already needed elsewhere on the machine.
Housing materials and the environment
The housing letter tells you the shape; the material tells you whether the unit survives the environment. The right choice is driven by moisture, chemicals, washdown and temperature, not by habit.
| Material | Strength | Corrosion | Use when |
|---|---|---|---|
| Grey cast iron | High | Low | General industrial, dry and indoor duty |
| Ductile iron | Very high | Low | Shock load, heavy duty, larger sizes |
| Pressed steel | Low to medium | Medium | Light duty, low cost, moderate speed |
| Thermoplastic | Medium | High | Washdown, chemicals, wet and hygienic areas |
Seal choice follows the same logic. A single-lip seal keeps dust out of a normal industrial environment; a contact seal with a flinger is used where water or fine slurry is present. In washdown and food areas the combination of a thermoplastic housing and a sealed insert is the standard answer, which is why the FYH, NTN and ANDGE unit ranges publish the sealing and material options alongside each size.
Relubrication fittings
Most pillow block and flange housings carry a grease nipple so a sealed-insert unit can be relubricated in place. The interval depends on speed, load and temperature, and the grease interval guide gives the method. If the housing has no fitting, the insert is sealed for life and should be replaced rather than opened. A greased unit running at moderate speed and temperature typically asks for attention every 2,000 to 5,000 h.
How to select and mount a mounted unit
Selection is a four-step decision, and mounting is where most units fail. Work through both in order.
- Match the housing to the mounting face. Base-mount positions take a P, PA or UKP; face-mount positions take an F, FL or FC; sliding belt adjusters take a T.
- Confirm bore and shaft condition. Measure the shaft, state millimetre or inch, and check that the seat is within the fit tolerance recommended for the lock type - see the shaft and housing fits guide.
- Choose the lock for the duty. Set screw for light steady duty, eccentric collar for reversing and vibration, adapter sleeve for heavy load and larger shafts.
- Choose the material and seal for the environment. Grease nipple for accessible positions, sealed for life where relubrication is impossible.
- Mount without distorting the insert. Seat the unit on a flat, clean face, tighten the mount bolts evenly, then slide the shaft through before locking.
- Lock in the direction of rotation. Turn an eccentric collar the way the shaft turns, then tighten to the torque stamped on the collar or given in the catalogue.
- Run the shaft and re-check the lock. After the first hour of running, confirm the lock has not crept and the shaft has not been scored.
If only the bore and the duty are known, the bearing selector filters the catalogue by housing type and size, and the glossary defines the unit terms. For the wider selection logic, the bearing type selection guide places mounted units alongside the enclosed radial families.
Common mistakes
- Locking a set screw onto a hardened shaft. The screw cannot bite, so it slips and the shaft is scored. Use an eccentric collar or an adapter sleeve instead.
- Omitting the locating pin or dowel. A unit that is only bolted can creep under vibration, and the resulting misalignment loads the insert.
- Over-tightening an eccentric collar. The inner ring distorts, the internal clearance disappears and the balls are pinched, which shows up as heat and noise.
- Bolting to a face that is out of square. The self-aligning seat is used up by the mounting error, so the shaft misalignment reaches the balls unopposed.
- Mixing metric and inch units on the same shaft. The bores are close but not identical, and a slip fit becomes a loose or seized fit.
Frequently asked questions
What does UCP stand for?
UCP is a pillow block mounted unit: U identifies the insert bearing series, C the cast housing, and P the pillow block shape. The trailing digits give the bore in millimetres, so a UCP 205 takes a 25 mm shaft. The same insert can also be supplied in flange, take-up or cartridge housings.
What is the difference between UCF and UCFL?
Both are flange units. UCF is a four-bolt square flange that spreads load over a larger pattern and resists rocking; UCFL is a two-bolt flange on a narrower footprint for positions where a four-bolt flange will not fit. The bore code is the same in both, so a UCF 205 and a UCFL 205 take the same 25 mm shaft.
Which locking method is best?
It depends on duty. Set screws are the quickest and cheapest and suit light steady load. An eccentric collar grips over a larger area and is the better choice for reversing drives and vibration. An adapter sleeve in the UK series is used for heavier loads and larger shafts because the taper generates a stronger, more even grip.
Can I replace just the insert bearing?
Yes, and that is the main advantage of the design. The insert is a separate self-aligning bearing that lifts out of the housing, so a worn unit can be rebuilt by fitting a new insert of the same bore and series. Confirm the insert series, the locking type and the seal before ordering, because the housing will accept more than one variant.
How much misalignment can a mounted unit take?
The spherical seat of the insert typically allows around two degrees of static self-alignment, enough to absorb shaft deflection and small frame errors. It is not a substitute for correct alignment: if the mounting face is badly out of square, the seat reaches its limit and the remaining error passes into the bearing.
Should I choose cast iron or thermoplastic housings?
Cast iron for dry, indoor and general industrial duty where strength and heat dissipation matter. Thermoplastic housings are chosen for washdown, chemical and wet environments because they resist corrosion and are lighter. The choice is driven by the environment and any hygiene requirement, not by the load alone.