A review on the design and operations challenges of a single toggle jaw crusher is presented. Strength and fracture toughness of the material to be crushed are intrinsic properties that determine the time and energy required to crush the material. Economy of the crushing process is partly dependent on the angle of nip. Productivity of the crusher can be improved upon by increasing the eccentricity of the eccentric shaft, use of reversible jaws, bush bearing and easily adjustable toggle plate. Vibrations and fatigue cracks in the crusher frame will be nipped in the bud through structural analysis at design stage. Determination of the optimal angle of inclination of the toggle plate, development of jaws with varying wear rate along the crushing chamber, and development of comminution energy models that take into cognizance relevant crushing parameters for simulation and optimization of the crushing process are some areas that require close attention.
A. Introduction
Crushing is the first mechanical comminution process after blasting of rocks and breaking of oversize rocks or boulders into crushable lumps. It involves reducing the lumps of rocks or ores into definite smaller sizes. Production of economically desirable sizes is the main objective in the aggregate industry, while liberation of the valuable minerals from the gangue is the ultimate aim in mineral processing. Crushers used in mining operations are commonly classified by the degree to which they fragment the feed material with primary and secondary crushers handling coarse materials and tertiary and quaternary crushers reducing ore particles to finer gradations.
The type of crusher to be used for a given job is dependent on the nature of material to be crushed, area of application of the material, maintenance and operational costs, power consumption, vibration, noise and environmental issues. More so, crushers can be classified on the basis of the breaking forces as:
(a) impact crushers e.g. hammers and rotor impactors;
(b) compressive crushers e.g. jaw, gyratory, cone and roll crushers.
Jaw crushers are commonly used as both primary and secondary crushers. Donovan noted that there are three types of jaw crushers, namely: Blake, Dodge and Universal jaw crushers; these are classified according to the location of the pivot point of the movable or swing jaw. The single toggle swing jaw is suspended on the eccentric shaft, which allows a lighter, more compact design than with the double toggle crusher. Moreover, the single toggle crusher is taking over most new applications due to lower cost and higher capacity; hence the need to carry out a wholistic review of the critical aspects of the machine that requires close attention during its design and operations stages; this is being done with a view to revealing ways to improve its performance.
B. Important design and operation factors of jaw crusher
There are some design and operation factors that must be given attention in jaw crushers at the design and operation stages, without which, the machine will not function economically. Such factors and critical components include: the nature of feed material, angle of nip, jaws, pitman, eccentric shaft, toggle plate, drawback rod, cheek plates, bearing, crusher frame, pulley and flywheel.
1. Nature of Feed Material
Crushers which comminute by compression are strongly recommended for hard, brittle and abrasive rocks, i.e., rocks with Mohs hardness value ranging from 6 to 7 and above. Donovan posited that the crushing force must exceed the fracture strength of a particle for it to fracture; however, rocks broken in jaw crushers fail at stress levels well below the compressive strength due to induced tensile stresses and the presence of cracks. It is the tensile strength of rock material that must be exceeded in order for it to fracture. Decrease in strength of rocks is due to pre-existing flaws and cracks within the rocks which act as stress concentrators as well as moisture effect on the mineral grains.
Physical properties of materials such as: moisture content, structure, friability, density, hardness and crushing strength are important design criteria, as these affect both the life of the liners and power requirement. Olaleye revealed that the higher the strength of a rock, the higher the crushing time under the influence of a crusher; this implies more wear to the crusher jaws.
2. Angle of Nip
The jaws are set at an acute angle to each other. This angle commonly known as the “angle of nip” is usually less than 26 degree. This is due to slipping effect when the angle is larger which reduces capacity. Niemela and Kieranen, stated that a desirable nip angle controls the ability to crush a given type of material at a commercial rate and it preferably falls between 17 degree and 27 degree. Exceeding the maximum angle causes regurgitation or slipping from the machine, while operating below the desired range leads to the production of undesirable dust and fines; hence, the machine tends to serve more like a pulverizer.
3. Crusher Jaw Plates
Compression of materials undergoing crushing in a jaw crusher is achieved when the movable jaw presses the feed against a stationary jaw. These jaws can be flat surfaced or corrugated. Crusher jaws were formerly made of white cast iron and later with high manganese austenitic steel also known as Hadfield steel, which is the dominant wear material for the jaws. Wear on these components increases as the feed lump is being reduced and moved towards the discharge.
According to Kinkel, the greatest amount of crushing is done at the lower edges of the jaws; consequently, these lower edges are subjected to much greater wear than any other part of the jaw, because, the greatest movement of the movable jaw is at the lower edge. These plates are made reversible, so that the worn end can be inverted to become the upper end of the plate, thereby, reducing the cost of replacing these worn jaws. The variation in the amount of wear on the fixed jaw calls for a variation in the surface hardness of the jaw. This will be difficult to achieve by casting, but can definitely be achieved by hardfacing techniques.
Additionally, virtual modeling results have shown that the strength to weight ratio of the movable jaw can be increased by increasing the number of stiffeners attached to its back. The fixed jaw is bolted to a support plate, while the movable jaw is bolted to the pitman for easy dismantling when they are worn out, to be replaced or partly worn, to be inverted.
4. Pitman
The pitman is journalled at the upper end to accept the eccentric shaft. This structure houses the eccentric lobe and supports the movable jaw. The lower end of the pitman is guided by the toggle plate and drawback rod attached to it. It has been demonstrated that a pitman with a cross-sectional support in the form of a honeycomb structure reduces or removes bending of the pitman and wear compared with a pitman without such support. The cross-sectional supports eliminate bending and distortion horizontally, with additional advantages including crushing material with smaller stroke count and smaller stroke length, reduction in the amount of energy required from the flywheel, lesser material requirement for producing the pitman, reduced mass of the pitman and avoidance of holes arising from casting, when open structured pitman is used.
5. Eccentric Shaft
Rotation of the eccentric shaft during operation by the pulley causes the movable jaw to make an elliptical movement. Increased eccentricity of the shaft leads to increase in throw; hence, increase in throughput capacity can be achieved without increasing the physical size of the jaw crusher by increasing the stroke of the eccentric shaft, decreasing the speed without increasing the crushing force through increased jaw width. Also, increased throw gives the advantages of retaining the structural design of the crusher and decreasing the machine loads. The crusher stroke is the displacement of jaw between the widest and narrowest points on an eccentrically gyrating cycle. Alternatively, Donovan defined the throw as the stroke of the swing jaw or the difference between the open side set and the closed side set. The open side set is the maximum discharge aperture, while the closed side set is the minimum discharge aperture.
6. Cheek Plates
The side or cheek plates are positioned on the left and right ends of the crushing chamber to prevent the material being crushed from reaching the frame of the crusher, which will lead to the wear of the frame. Cheek plates are also made of manganese steel; materials such as white cast iron and hardfaced steel can be used since the impact on the side plates is minimal compared to the stationary jaw. Worn cheek plates together with worn jaws should be replaced on time as worn chamber will affect the capacity of the crusher, size and shape of the produced particles.
7. Toggle plate
The toggle plate is used to hold the lower part of the jaw in position; this depends on the desired product size. Toggles are designed to be adjustable for easy removal of uncrushable object such as tramp iron and to achieve proper discharge setting. Mechanisms for toggle adjustment include: spring relief mechanism for relieving strain on the jaws when tramp iron lodges between the jaws, shims and hydraulic cylinders, which allow easy adjustment of discharge setting by moving the toggle block to the desired setting, remote controlled electromechanical actuation mechanism is possible. A trial mechanism design with different acute angles of inclination showed that the throw is not only related to the eccentric shaft, but also the toggle inclination. Considering the role of the toggle plate, there is need to determine the optimal angle of inclination for efficient performance.
8. Drawback or Tension Rod-Spring Mechanism
The drawback rod is attached to the lower end of the movable jaw or the pitman carrying the movable jaw, and carries a spring at the opposite end. The rod-spring subassembly retrieves the movable jaw from the furthest end of travel. Here, the spring deflection and the rigidity of the rod are pertinent. This spring-biased rod facilitates the cyclical return of the lower end of the jaw to the base position .
9. Pulley and Flywheel
The weights of these two machine elements need be balanced as any deviation may lead to undesired twisting of the eccentric shaft and increased vibration. They are firmly keyed to the opposite ends of the eccentric shaft. Usually, they are made of gray cast iron because of its good vibration damping, machinability and resistance to sliding wear. The pulley has two or more grooves and is driven by belts attached to the prime mover which may be a combustion engine or an electric motor. The flywheel supplies the moment of inertia of a system, as it serves as a reservoir, which stores energy during the period when the supply is more than the requirement and releases it when the energy requirement is more than the supply. Hence, the inertia required to crush a material in a jaw crusher is provided by the flywheel.
10. Bearing
Bearings hold the eccentric shaft in position and enable its free rotation. Lubrication of these elements with grease and sealing of their ends should be ensured to prevent entry of dust. Dynamic loads from the pitman, movable jaw, flywheel, pulley, drawback rod, toggle plate and eccentric shaft lead to severe wear in the bearings. Replacing a roller bearing with bush/babbited bearing in two halves, increased the availability of a jaw crusher by 17%, reduced breakdown and mean time to repair by 89%, while the maintenance was reduced by 86%. Also, this increased customer satisfaction and enhanced productivity as removal of the pulley and flywheel before changing the bearing was avoided.
11. Crusher Frame
Strong foundation to accommodate vibrations arising from the alternate loading and release of stresses in the jaw crusher has been advocated by Zeng and Forssberg. High dynamic forces are present during operation of crushers. The entire load is transferred to the supporting structures and foundations during operation. Loads on the crusher frame must be taken into consideration at the design stage and attention should be given to modal parameters of the structure to prevent possible resonance problems.
Finite element method has been used to identify the cause of resonance and fatigue cracks in a jaw crusher structure. Additional side bracings and replacement of existing longitudinal bracings resolved these problems. The key to this solution method is that incorporating diagonal bracing inside the frame increases the stiffness, which results in the increase in natural frequency. The problems of vibrations and fatigue cracks underscore the relevance of proper structural analysis at the design stage, prior to construction and operation.
C. Review of jaw crushers’ design models.
The major problem in the design of comminution equipment is that most of the energy input to a crushing or grinding machine is absorbed by the machine itself and only a small fraction of the total energy is available for breaking the material. All the theories of comminution assume that the material is brittle. The cost of power is a major expense in crushing, so the factors that control cost are to be given important considerations. An ideal crusher would have a large capacity; require a small power input per unit of product and yield a product of the single size distribution desired.
In virtually all size reduction machines, the breakage forces are either by compression or impact. The jaw crushers cause fracture by compression, since this is the most practical method of applying a fracture force to large particles. The reduction ratio is a measure of the extent of size reduction, and is defined as the ratio of the feed size to the product size. Single toggle jaw crushers have sizes ranging from 125 x 150 mm to 1600 x 2100 mm; power requirements from 2.25 to 400 kW; speed from 120 to 300 rpm; and reduction ratio from 4:1 to 9:1.
D. Conclusions
The major challenges faced by the single toggle jaw crusher emanate from the nature of the material to be crushed, angle of nip and components design. Strength and toughness of a rock are directly proportional to the comminution energy; hence, a rock particle with higher strength and toughness will require more time and energy to break under the influence of a jaw crusher. Angle of nip must be maintained between 17 degree and 27 degree as deviation from this range leads to undesirable and uneconomical output.
The diametric configuration of the eccentric shaft is the key to achieving better throw; consequently, increase in the throughput of the crusher can be achieved by increasing the eccentric lobe rather than increasing the width of the jaw. Wear on the jaw plates is more on the fixed jaw and increases as the material being crushed moves towards discharge. Reversible jaws have been used to reduce frequency of repair or replacement. Hard facing technique has been suggested as a means of taking care of the wear variation. Stiffened jaws give high strength to weight ratio and may require less energy. Honeycomb-structured pitman increases horizontal rigidity, demands less energy from the flywheel and accomplishes crushing task with less stroke count and length.
A toggle mechanism that will guarantee quick and easy removal of tramp iron without breaking the plate is still needed in the jaw crusher. Crushing force is derived from the action of the toggle plate and the inertia of the flywheel whose weight is counter-balanced by the pulley. The use of bush bearing reduces the mean time to repair and increases availability, as dismantling of the pulley and flywheel is avoided when repairing or replacing the bearing; this does not only increase customer satisfaction and productivity, but also reduces maintenance cost. Application of friction geomodifiers to worn parts such as bearings will reduce frequency of shutdown and maintenance cost and increase the service life of the equipment.
Proper structural analysis of the crusher frame should be carried out at design stage to prevent vibrations and fatigue cracks during operation. Power required to crush a material as given by Bond’s equation did not include the mechanical power required to drive the movable jaw and toggle plate. The mechanical power need to be added to that of comminution prior to selecting a prime mover. Existing crushing power models are not all encompassing as important parameters as fracture toughness, compressive strength, reduction ratio, angle of nip, work index and jaw dimensions were not integrated in one model; hence, there is need to develop complete power model for the purposes of simulation and optimization of the jaw crusher, whether laboratory or industrial.
