Mechanic-powered system

Designations
The "blast wheel" is the name given to the mechanic projection device which fit out automatic machines with high output, without any operator. These machines are often called by the users "shot blast machines" because this process uses even exclusively metallic shots.

Principle
The blast wheel is a wheel fitted with pallets, which transmits to an abrasive flow a kinetic energy stemming from the mechanical energy it receives from a driving motor.
The abrasive or shot flow is introduced into the wheel centre and taken by the inner extremity of the pallets. (Picture 1)
Carried by the wheel rotation and subjected to the centrifugal force effects, the abrasive grains go away from the centre along the pallets, up to the outer extremity and are projected in a spray pointed towards the part to be treated (Picture 2).

Blast wheel features

Projection
During the rotation, the way covered in the air by each grain along the pallets takes the shape of a spiroid curve.
When the grain leaves the blade, it does it according to a trajectory tangent to this curve. The vast number of trajectories generated by all the grains forms what is called the blast wheel spray which maximum intensity is situated around the centre where the projection distance is shorter. (Picture 2)

Projection intensity
The intensities vary according to the mass of the projected grains and the projection speed of these grains (mV2), and depend thus on the abrasive granulometry and the blast wheels speed of rotation (see heading parameters).
- the blast wheels speeds of rotation, according to the rotors dimensions (diameter 250 to 650 mm), range from about 1200 to 4000 revs/min.
- the projection speeds vary from 50 to 90 m/sec.

Output and power
The classical blast wheels tolerate very important shot flows in relation to the implemented power.
In average and according to the type, 10 to 13kg of steel or cast iron shots are projected in one minute, while absorbing a 0.75kW power for a projection speed of 75 m/sec.
According to the applications and the output wanted, the shot blast machines can be fitted with blast wheels with a power reaching 75 kW and with an output of 1300 Kg of shot per minute.

Ccontitutives elements of blast wheel

The set
The blast wheels differ from one to the other by their injection mode and their shape, the pallets number or arrangement.
The constitutive elements are assembled on a base and are made of:
- a shaft assembled on balls bearing and which receives the rotation movement of a motor and supports the blast wheel rotor composed of one or two supports trays.
- a set of high hardness cast iron projection pallets fixed on the rotor.
- a splash guard-carter which contains the rotor and supports the injection system and the feeding nozzle.

Feeding method
The rotor feeding with shot or abrasive and the injection are ensured by 3 systems:
- gravity: the pallets are fixed tangentially to a circle on a unique tray and are fed thanks to gravity trough the nozzle.
- injection: the abrasive is injected in a nozzle through a warm-air super pressure machine, or through compressed air directly to the pallets centre.
- pre acceleration: the abrasive is pre accelerated mechanically by a distributor (additional blast wheel) situated inside the injection cage in the centre of the rotor.
It goes away and is immediately taken back by the pallets, through an opening situated in the injection cage, whose positioning determines the spray direction. This is the most widespread system.

Applications

The blast wheel can be assembled alone or in a group on a shot blast machine.
The wheel shot blast machines are particularly adapted to the treatment of great quantities of similar parts (tumble barrel; rotary table; metallic band which turns upside down or suspended clusters) and to the treatment of long structures which can file past continuously through tunnel machines.
The wheel is generally used to project steel or cast iron shots.
However, in some applications such as shot peening or surface finishing, it is used with ceramic beads, or for thermo hardening removing of rough patches operations, with plastic abrasives.
It is never used with abrasives such as aluminium oxide or glass beads, because the first ones generate a too fast wear and the second ones are subjected to a fragmentation too important.