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Its all about the pressure

Here we cover how the machine ups the incoming pressure of your water supply to achieve that perfect creamy espresso.
To get water through your finely pressed grounded coffee requires quite a bit of pressure, typically around the 9 bar mark (130psi) for the perfect espresso.

Many years ago at the dawn of espresso machines, this was achieved using pistons, springs and levers that had to be manually operated, thankfully, the baristas work has been made a lot easier with the use of electrically operated pumps.
These pumps come in 2 forms and mainly fall into the following sectors of operation:

Rotary Vane - this dominates the commercial sector due to its long life and smoother operation.

Vibratory Pump - found extensively used in the domestic market and some compact commercial machines.
There are advantages to using both types of pump. Vibratory pumps are physically smaller and cheaper, and also easier in the most to replace. 

Rotary pumps offer quieter and a more consistent pressure regulation during operation, and when using filtered water they also offer a longer life span than the vibratory type.

Side by Side

ROTARY VANE PUMP

A rotary pump is a mechanical device driven by an electrical motor. It also contains quite a few complex components to achieve the correct and constant pressure to the group heads. To achieve this, the motor spins a series of vain's that are offset inside a central chamber. As the vain's rotate against the wall of the chamber, incoming water is compressed and thus pressurized before exiting the pump chamber. The pressure can be adjusted (within limits) by means of an adjustment screw mounted on the side of the pump body.

For: Stable pressure with quieter operation, longer lifespan.
Against: Physical size where space is tight.

VIBRATory PUMP

A vibratory pump or vibration pump, is generally a small electromagnetic device. This operates using a piston which is attached to a magnet and set inside an electrical coil. When current flows through the coil, this causes the magnet to rapidly move the piston back and forth, this action then pushes water through the machine. Because this operation works on your mains electric supply, the oscillations can be 50 or 60hz, hence the rotary vane pump has a smoother operation.

For: Compact size, relatively cheap.
Against: Noisy operation, can be short lifespan.

Faults & Possible Cures

Excessive Group Head Pressure
Normally down to the by-pass valve being stuck in the closed position. Some pump suppliers can supply just the regulator assembly, but more often than not, a replacement pump is the order of the day.
Lack Of Group Head Pressure
Most possible cause would be a seized pump, with the pump disconnected from the motor you could see if the motor turns when you run the groups. If the motor runs normally then the fault is with the pump. If the motor is not running without the pump connected, then the fault lies with the motor, motors are pretty tough and rare to fail, so check the electrics are correct before replacing the motor.
Erratic Group Head Pressure
Faulty operation on the 'by-pass valve'. You can try cleaning the pressure regulator assembly, or replace the pump.
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