Flange and Coupling Compatibility When Replacing a Press Brake Hydraulic Unit Electric Motor

When the motor of a press brake's hydraulic unit is replaced, flange and coupling compatibility is checked at three points: the new motor's flange must seat in the spigot recess and bolt holes of the existing pump bell housing, the shaft diameter and key must fit the coupling hub, and the shaft length must bring the coupling halves together at the correct axial gap. If any of these three does not match, the bell housing, coupling or an adapter flange changes. This article explains how these checks are carried out on site for layouts typical of press brake units, how to assess the flexible coupling and when a standard motor cannot take the old one's place.

How is the motor arranged on press brake units?

On press brakes, the hydraulic unit is usually on a tank placed on the side frame or at the back of the machine. Three layouts are most common on site:

• Vertical on top of the tank (V1, shaft down): The motor stands upright on the tank lid; the pump hangs into the oil below the lid together with the bell housing. It is common because it is compact and quiet. The motor flange carries the weight of both the pump and the bell housing.
• Horizontal on top of the tank (B35 or B5): The motor is horizontal; the bell housing and pump are attached to the flange. With a foot-and-flange motor, the feet transfer the load to the base.
• Oil-immersed motor-pump unit: In some compact units the motor is also in the oil. The motor in this design is a special motor built for operation immersed in oil.

Oil-immersed motors and pump systems driven by servo motors may fall outside the scope of the standard series in HEM Motor's 2026 catalogue; for such units, ask us with the nameplate and application details. The checks below apply to conventional units in which the motor stands outside the tank and runs at fixed speed.

Flange compatibility: what information is taken from the old motor?

The name of the flange alone is not enough. Two "B5 flange" motors may have different flange sizes. Before the old motor is removed, record the following:

• Flange type: If the holes in the flange go right through (the bolt passes through from behind the flange), it is the large flange, corresponding to the B5 arrangement. If the holes are threaded (the bolt is screwed in from the bell housing side), it is the small flange, corresponding to the B14 arrangement. Both are found on press brake units.
• Spigot diameter: The diameter of the raised part of the flange that fits into the bell housing. Because the bell housing is centred on this diameter, the dimension must be exactly the same.
• Pitch circle and number of holes: The diameter of the circle passing through the centres of the bolt holes, and the number of holes.
• Nameplate information: The IEC frame code (e.g. 132M) and, if present, the flange code. If the nameplate does not show a flange code, the measurement is the reference.

Mounting arrangements are offered according to the frame; the flange size is confirmed with the order. Sending the values above in writing with the order therefore eliminates the risk of a motor arriving with the wrong flange. Flange and shaft dimensions by frame are given in the 2D technical drawings and the 2026 technical catalogue; the mounting options are summarised in the question on mounting arrangements.

Coupling compatibility: before reusing the flexible coupling

On press brake units, the motor and pump are usually connected by a jaw coupling with two metal hubs and a flexible insert between them (commonly called a "spider" or "star"). When replacing the motor, the coupling is assessed in three respects:

• Motor-side hub: The hub's bore and keyway must fit the new motor's shaft. With a new motor of the same frame code this is usually the case; if the frame changes, a new hub is needed. The hub is not hammered onto the shaft; it is fitted by heating or with a puller.
• Flexible insert: A hardened, cracked or crushed insert increases vibration and noise. Replacing the motor is the right moment to renew the insert. The material and hardness of the insert are selected according to the coupling manufacturer's table.
• Hub position: The axial gap specified by the coupling manufacturer must be left between the two hubs. If the hubs butt against each other, axial load acts on the pump shaft bearing; if they are too far apart, the jaws do not engage sufficiently.

Shaft length and bell housing depth: an often-skipped check

If the new motor's shaft length differs from the old one, the point where the two hubs meet inside the bell housing shifts, even if the motor hub is fitted at its old position on the shaft. A practical way to check this on site is:

• Before removing the old motor, measure the depth from the bell housing flange face to the front face of the pump hub.
• On the old motor, note the length from the flange face to the shaft end and the position of the hub on the shaft.
• Take the same measurements on the new motor; fix the hub at a position that leaves the gap specified by the coupling manufacturer between the two hubs.
• On a vertical unit, the hub's set screw or key must be tightened before the motor is lowered into place; otherwise the hub can slide on the shaft.

If the new motor has a different frame code (e.g. moving from 132 to 160 because of higher power), the bell housing does not fit directly. In that case either a new bell housing or an adapter flange between motor and bell housing is needed; because the adapter flange also changes the shaft length, the coupling hub position is recalculated.

Power, speed and the press brake operating pattern

On a press brake the motor usually runs continuously throughout the shift; the pump is loaded while the ram descends and bends, and runs idle while waiting. The high-pressure bending phase is short, but some jobs also involve dwelling under pressure for longer. Motor power comes from the pump flow and maximum working pressure, and the machine manufacturer's value is the reference; power is not estimated from the press brake's tonnage.

Most hydraulic pumps run with a 4-pole motor, i.e. at 1,500 rpm synchronous speed; the number of poles is not changed on the new motor. Star-delta starting requires the unit to start unloaded. The motor's direction of rotation is checked against the arrow on the pump with a short trial at the first start.

Example 4-pole models in the power range found on press brake units:

• 132S-4 aluminium: 5.5 kW (7.5 HP), 1,460 rpm, IEC 132S, 400/690 V (Δ/Y), 10.7 A. Its light frame reduces the load on the flange in vertical tank-top mounting.
• 132M-4 cast iron: 7.5 kW (10 HP), 1,460 rpm, IEC 132M, 400/690 V (Δ/Y), 14.3 A, 49.1 Nm.
• 160M-4 cast iron: 11 kW (15 HP), 1,465 rpm, IEC 160M, 400/690 V (Δ/Y), 20.4 A, 71.7 Nm.

Review suitable HEM Motor models and current prices on the buy electric motors page. General information specific to hydraulic applications is on the hydraulic pump motors page.

Hypothetical example: noise and vibration in the unit after replacement

This example has been constructed for explanation. Suppose the 7.5 kW, 132M-frame motor on the vertical tank-top unit of a press brake has been replaced, but from the first day noise and vibration in the unit have increased.

• Inspection shows that the new motor's hub has been fixed a few millimetres further in on the shaft than the old position, and the two hubs are butting against each other.
• It also turns out that the old, hardened flexible insert was reused.
• The hub is brought to the correct axial gap by measuring the bell housing depth and shaft length, and the insert is renewed. Noise and vibration return to normal; no problem is found in the motor.

Conclusion: information to prepare before replacement

The success of a motor replacement on a press brake unit depends on transferring the flange, shaft and coupling dimensions correctly. Send the following with the order:

• A photo of the old motor's nameplate (power, speed, voltage, frame code)
• Mounting position (vertical V1, horizontal B5 or B35) and flange type (through holes / threaded holes)
• Flange spigot diameter, pitch circle and number of holes
• Shaft length, bell housing depth and coupling hub details
• Pump type, maximum working pressure and starting method

You can see the models and current prices on our shop page. Share a photo of your current motor's nameplate, its speed and mounting details via the nameplate submission page and request a quote for a suitable model. The contact page can also be used for your questions.

Frequently Asked Questions

How do I tell a B5 flange from a B14 flange on a press brake unit?

If the flange holes go right through, it is the large flange (B5 arrangement); if they are threaded, it is the small flange (B14 arrangement). Still confirm the dimensions with the spigot diameter and pitch circle.

Can I use the old coupling on the new motor?

The metal hubs can be used if they fit the shaft diameter; the flexible insert should be renewed if it is hardened or cracked. Adjust the hub position to the new shaft length.

If I fit a more powerful motor, will the pump bell housing fit?

If the power increase changes the frame code, the flange and shaft also change; the bell housing will not fit directly. A new bell housing or an adapter flange is needed.

Can a standard motor be used for a servo-driven press brake pump?

Servo pump systems use special motors designed together with the drive; they are not within the scope of the standard series. Ask us with the nameplate and system details.