Mounting is the part of motor selection that gets settled last and causes trouble first. The electrical rating is negotiated carefully, then someone picks a mounting because it is what the previous machine used, and eighteen months later a bearing fails early and nobody connects the two events.
This is a short tour of the standard configurations, what each one does to the load path, and the mistakes that cost bearings.
The Three Standard Geometries
B3 – Foot Mount
The motor sits on machined feet bolted to a base plate, and the shaft drives through a coupling, belt or chain. Load is carried by the base, not by the motor housing. B3 is the most forgiving arrangement for heavy or shock loads and the easiest to shim, but it consumes the most space and it puts full responsibility for shaft alignment on the installer.
B5 – Flange Mount
A large machined flange with through-holes bolts directly to the driven machine, and the motor hangs from that face. The register spigot locates the shaft concentric with the housing bore, so alignment is largely built in. B5 is compact and repeatable, which is why it dominates gearbox and pump integration. The trade-off is that the flange and its fasteners now carry the motor's weight plus any dynamic load.
B14 – Face Mount
A smaller flange with tapped holes, bolted from the machine side. Mechanically similar to B5 but lighter, more compact and rated for lower loads. B14 suits small drives where space is tight and the motor mass is modest.
Our standard PMDC frames are available in all three. Dimensional tables for the A60, A80, A90 and larger series, including register diameters, pitch circle diameters and shaft data, are published on the PMDC motor product page, and the geared variants are on the geared motor range page.
Choosing Between Them
| Factor | B3 Foot | B5 Flange | B14 Face |
|---|---|---|---|
| Alignment responsibility | Installer | Built into register | Built into register |
| Load capacity | Highest | High | Moderate |
| Space envelope | Largest | Compact | Most compact |
| Typical use | Belt and chain drives, heavy loads | Direct gearbox and pump coupling | Small integrated drives |
| Serviceability | Easy to shim and realign | Remove as an assembly | Remove as an assembly |
Vibration and Alignment by Mounting Type
Each geometry fails in a characteristic way.
Foot-mounted motors suffer from soft foot: one foot does not sit flat, the bolts pull the housing into a twist, and the bearing runs with a preload it was never designed for. Check all feet on the machined base with a feeler gauge before torquing, and shim rather than forcing.
Flange and face mounted motors transmit driven-machine vibration straight into the motor housing because there is no compliant path between them. Where the driven load is unbalanced, that vibration goes directly through the bearings. A flange mount also makes the motor a cantilever, so any overhung load on the shaft is amplified at the bearing nearest the flange.
All three are damaged by the same underlying error, which is asking the motor bearing to carry a load it was not sized for.
Four Mistakes That Cause Premature Bearing Wear
- Overhung load from a belt or sprocket too far out on the shaft. Bearing load rises with the distance from the bearing to the load. Move the pulley as close to the shaft shoulder as the design allows.
- Over-tensioned belts. A tight belt feels safe and is a permanent radial load on the front bearing. Tension to specification, not to feel.
- Rigid couplings across a misaligned pair. A rigid coupling does not absorb misalignment, it transmits it. Use a flexible coupling unless the alignment is genuinely controlled.
- Ignoring the mounting orientation. A motor specified for horizontal running and installed shaft-up can see thrust and lubrication conditions it was never rated for. Tell your supplier the orientation.
Bearing and brush wear symptoms and how to read them are covered in our DC motor maintenance guide and in more depth in common DC motor problems and preventive maintenance.
Mounting a Geared Motor or Transaxle
Once a gearhead is fitted, the gearbox output, not the motor, is what bolts to the machine, and the gearbox has its own radial and axial load limits. Our PMDC geared motors use parallel-exit steel gears with exit axles up to 20 mm in AISI 303, and the PMDC transaxle range integrates the differential and axles into the drive unit, which changes the mounting problem entirely. Sizing guidance is in selecting gear ratio and torque for custom geared PMDC motors and the geared PMDC motor selection guide.
Get the Drawing Before You Finalise the Chassis
The cheapest moment to fix a mounting problem is before the chassis is drawn. We issue CAD-generated dimensional drawings with preliminary quotations, so your mechanical team can check register diameter, bolt pattern, shaft extension and overall length against the enclosure before anything is cut. If you are preparing that enquiry, the specification checklist for a first custom motor call lists exactly which mounting dimensions we need from you, and customisation in PMDC motor design covers what else can be changed at the same time.
Frequently Asked Questions
Q1. What is the difference between B5 flange mount and B14 face mount?
Both bolt the motor directly to the driven machine through a machined flange with a locating register. A B5 flange is larger with through-holes and carries higher loads. A B14 face is smaller with tapped holes, is more compact, and is rated for lighter duty.
Q2. When should I use a foot-mounted (B3) motor?
Choose B3 when the drive is by belt or chain, when the load is heavy or shock-prone, or when you need to shim and realign in service. The base plate carries the load rather than the motor housing, which is the most forgiving arrangement.
Q3. Which mounting type is best for a gearbox?
Flange mounting is normal for direct gearbox coupling because the register holds the motor shaft concentric with the gearbox input, removing most alignment error. On an integrated geared motor the gearbox housing becomes the mounting interface.
Q4. What causes premature bearing failure on a motor?
Most commonly an overhung load mounted too far from the bearing, an over-tensioned belt, a rigid coupling across a misaligned shaft pair, or soft foot on a foot-mounted machine where the housing is twisted by the hold-down bolts.
Q5. Does mounting orientation matter?
Yes. A motor specified for horizontal running and installed shaft-up or shaft-down can see thrust loads and lubrication conditions outside its rating. Always state the intended orientation when specifying.
Q6. Can Globe Scott Motors supply a custom mounting configuration?
Yes. B3, B5 and B14 are standard across our PMDC frames, and shaft diameter, length, keyway and flange details can be cut to your drawing. CAD dimensional drawings are supplied with the preliminary quotation.
Q7. What mounting dimensions should I send with an enquiry?
Register (spigot) diameter, bolt pitch circle diameter and hole size or thread, shaft diameter and extension length, keyway details, maximum overall length available in your enclosure, and the mounting orientation.
Conclusion
Mounting configuration decides who owns shaft alignment, how vibration reaches the bearings, and how much space the drive consumes. Foot mounting gives you adjustability and load capacity, flange and face mounting give you built-in concentricity and a smaller envelope.
Whichever you choose, settle it against a real dimensional drawing before the chassis design is frozen. It is the cheapest correction you will ever make.
Need a mounting drawing before you finalise your chassis?
Tell us your envelope and interface. We will send CAD dimensional drawings with the preliminary quotation.