These are slow speed machines for coarse size reduction of large quantities of solids. The main type of crusher are jaw, gyratory, Roll and toothed rolls crushers. The first three types operate on compressive force and can crush very hard and brittle rocks under primary stages of crushing operation. The primary stage of crusher mainly two types

1. Black jaw crusher
2. Dodge jaw crusher

Jaw crushers :

According to working principle mechanism point of view jaw crusher is two type black and dodge. Through the working principle these crushers mag be different constructional point of view, they are almost identical excepting two notable different.

Construction of Black and Dodge Crusher :

● As the name suggest a jaw crusher has two jaws set to form a V-shape at the top through which feed is admitted in to the jaws. One of the jaw is fix to the main frame of the crusher almost vertically while the other one is movable. The swing jaw reciprocate in a horizontal plane and makes an angle 20 to 30° with the stationary jaw and is driven by an eccentric so as to that it applying a great compressive force on the ore lumps caught between the jaws.

● The crushing faces are made from hadfield manganese steels when extensive wear is observed on any of the faces it is replaced by a new one. The crushing jaw faces are flat or slightly sluged or may carry shallow grooves.

● Initially the large lump is caught at the top and is broken. The broken fragments drop to the narrower bottom space and is again recrushed when the jaws close in next time.

Working Principle :

Black jaw crusher -

● In the machine an eccentric drives the piman. The circular motion of the main shaft is converted to up and down motion of the pitman via the eccentric and finally the up and down motion is converted to and fro motion with the help of two toggles.
Working of jaw crusher
Black jaw crusher

● One toggle is fixed to the main frame and pitman while the other one is fixed to moving jaw and pitman from mechanical stand piont, toggles are the weakest member of the jaw crusher.

● When accidentally large stresses are set up in the jaws due to the extremely hard material entering in to the jaws, the toggles fail first preventing the breaking of vital parts of the crusher.
Black jaw
Black and dodge jaw crusher
Jaw crusher

● The other important components of the black jaw crusher is the fly wheel which is fitted on to the main shaft. The crushing action is only possible during the forward Stoke.

● To equalize this uneven load, one number of flywheel are used on the main shaft. On back stroke, the material that has been crushed is allowed drop freely through the jaws.

Dodge Crusher - 

● Look wise it is almost similar to black crusher here moving jaw is pivoted at the bottom, so maximum movement at the top. The width of feed received opening gape is variable while width of the discharge opening is fixed. Due to the fixed discharge opening the product is more uniformly sized.

● The inherent problem with this crusher is it's tendency to choke and is why it is less widely used. This crusher is usually made in smaller size than the black crusher because of high fluctuating stresses working on the members of the machine.

Characteristics of Jaw Crusher :

The crusher are intended for the use as primary crusher that is to receive coarsest lump jaw crusher have the gape. The length of receiving opening is some what greater than the width. The crusher have a varing width of discharge opening which is known as set. The set determine the product particle size.

Reduction Ratio -

RR = avg. Feed size / avg. Product size
This is very important parameters for determined the energy consumption.

Capacity - 

It capacity depends on the length and width of the receiving opening and width of discharge.

T = 0.6LS, where
T = capacity Ton/hr
L = length of opening in inch
S = set or width of discharge in inch.

Energy consumption - 

It depends on 1. Size of feed, 2. Size of product, 3. Capacity of the M/c, 4. Properties of rock that is hardness, specific gravity etc.
Here 10-20% of the total input energy is consumed for operation another lost in the machine, total consumption of 0.3 - 1.5 kWh/t of ore crushed.

Efficiency of jaw crusher -

It efficiency can be increased a little bit by analysing the some point. It also depends on the physical properties of the ore to be crushed.
1. Specific gravity of the ore.
2. Hardness of ore
3. Moisture content in the ore
4. Structural weakness planes of the ore.

Difference Between Black Jaw and Dodge Jaw Crusher : 

Black Jaw Crusher

1. It has got two toggles.
2. It has one pitman
3. It's movable jaw is pivoted at the top, so has variable product discharge opening while feed received opening is fixed.
4. No choking takes place here as it has variable discharged. It operate on principle of force feed.
5. Black jaw are mechanically more balanced so very large capacity black crusher are used.
6. Product size is large and more fine.
7. It is preferred in large industrial set up where elaborate screening and other size reduction carried out.
8. High cost of m/c.
9.  Less breakdown.

Dodge Jaw Crusher

1. It has only one toggle.
2. It has no pitman
3. The movable jaw is hinged at the bottom so the discharge opening is fixed. The set is fixed, this result in almost uniform sized product.
4. Chocking of discharge is very common problem as the set is quite small as compared to receiving opening.
5. Mechanically the design of crusher is inferior. So they are built only to low capacity.
6. Product size distribution is more uniform.
7. It is preffred where jaw crusher is to be used as only size reduction proposed.
8. Low cost of the m/c.
9. Higher breakdowns.

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