Why Does The Phenomenon of Unstable Contact Occur When Installing New Movable Jaw Plates?

1、 Overview of phenomena

In the daily maintenance and accessory replacement of jaw crushers, unstable contact when installing new movable jaw plates is a common installation problem. The so-called unstable contact refers to the failure to achieve good comprehensive contact between the installation fitting surface of the movable jaw plate and the installation base surface of the movable jaw body, resulting in local gaps or uneven fitting.

This kind of unstable contact can bring a series of adverse consequences. When the fit between the jaw plate and the machine surface is not tight, the huge impact force generated during the crushing process cannot be evenly transmitted to the machine body, resulting in local areas of the jaw plate being subjected to stress beyond the normal range, thereby accelerating wear in that area and even causing early fracture of the jaw plate. At the same time, unstable contact can also cause unnecessary vibration and abnormal noise in the jaw plate during operation, affecting the normal operation stability of the equipment.

2、 Analysis of the main reasons for unstable contact

2.1 Shrinkage deformation during casting and heat treatment process

The movable jaw plate is often cast from wear-resistant materials such as high manganese steel (such as ZGMn13). The linear shrinkage rate of high manganese steel is relatively high, and there will be some shrinkage deformation during the casting solidification process. In addition, high manganese steel castings also need to undergo water toughening treatment - heating to 1050-1100 ℃ and rapid cooling - to eliminate as cast carbides and obtain a single austenitic structure. This heat treatment process may also cause certain dimensional changes and warping deformation in the castings. Due to the complex structure and uneven wall thickness distribution of the jaw plate, it is difficult to achieve complete dimensional consistency during casting and heat treatment, and there is inevitably a certain degree of flatness error in the installation and fitting surface of the finished jaw plate.

2.2 Uneven machined surface

The installation and fitting surface of the movable jaw plate needs to be machined to ensure a certain flatness. However, in actual production, there are differences in machining accuracy among jaw plates from different manufacturers and batches. The installation surface of some jaw plates may have local protrusions, indentations, or excessive roughness, which may result in the inability to achieve a tight fit with the installation base of the moving jaw body after installation.

Similarly, the moving jaw body of a jaw crusher may experience wear, local damage, or deformation on its installation base during long-term use. When the installation base of the new jaw plate is matched with the old body, the shape and position errors of the two overlap, making the problem of unstable contact more prominent.

2.3 Unclean residues on the contact surface

Before installing the new jaw plate, if the installation base of the moving jaw body is not thoroughly cleaned, residual material particles, rust, oxide scale or casting burrs on the contact surface can cause the jaw plate to be partially suspended after installation, resulting in stress concentration when subjected to force. This seemingly minor negligence can become a significant factor in unstable contact.

2.4 Improper installation and fastening operation

The movable jaw plate is fixed to the movable jaw body by multiple bolts. If the tightening sequence is improper, such as fully tightening one bolt at a time and then tightening other bolts, it can cause uneven force on the jaw plate, resulting in deflection and tight contact in some areas while gaps exist in other areas. In addition, factors such as inconsistent bolt torque and insufficient anti loosening measures can also exacerbate the problem of unstable contact.

2.5 Assembly method and parallelism requirements not met

According to industry standards, the tooth profiles of the movable jaw plate and the fixed jaw plate should be in a basic meshing state (with tooth peaks aligned with tooth grooves), and the two should maintain appropriate parallelism. When installing a new jaw plate, if auxiliary tools such as copper, lead, zinc, etc. are not used to calibrate the parallelism, the jaw plate may experience relative sliding during operation, further exacerbating the unstable contact.

3、 Solution and operational points

In response to the above reasons, the following measures can be taken in practical operation to solve the problem of unstable contact of newly installed movable jaw plates.

3.1 Preparation before installation

Cleaning the contact surface: Before installing the new jaw plate, the installation surface of the movable jaw body should be thoroughly cleaned to remove residual material particles, rust, oxide scale, and casting burrs. Tools such as scrapers, wire brushes, or angle grinders can be used to ensure that the contact surface is clean and free of obvious protrusions or indentations.

Check the flatness of the contact surface: Inspect the installation base of the frame and movable jaw to ensure that their surfaces are flat. If there are obvious protrusions or depressions, grinding treatment should be carried out.

3.2 Pad Plastic Materials - Core Measures

A commonly used and effective method to solve the problem of unstable contact is to place a layer of plastic material between the jaw plate and the installation base of the moving jaw body. The principle of this approach is that plastic materials can deform under the pressure of bolt tightening, filling the small gaps between the contact surfaces of the two, thereby evenly distributing the pressure and eliminating local stress concentration.

The commonly used plastic cushion materials include the following:

Lead plate: With a soft texture, good plasticity, and moderate price, it is a commonly used cushion material. The thickness of the lead plate is generally controlled between 3-5mm, and can be adjusted appropriately according to the actual gap situation.

Zinc alloy plate: has a certain strength and good plasticity, suitable for installation occasions with high requirements.

Plywood: Wood material, economical and practical, suitable for situations with large gaps, but with relatively limited durability, suitable for the installation of temporary or small-sized equipment.

Cement mortar: For larger gaps, cement mortar can be poured to fill them and provide stable support after the cement solidifies.

Copper skin: can be used for fine adjustment of local gaps, and also has the function of assisting in calibrating parallelism.

In practical operation, the thickness of the cushion material should be selected according to the specific situation of the contact surface gap. Normally, it is recommended to use cushioning materials or plastic materials with a thickness of 5-10mm. If the gap is too large, it can be considered to use multi-layer pad stacking or replace the jaw plate for re matching.

3.3 Key points for tightening bolts

Symmetrical step-by-step fastening: After placing the cushion material, first place the movable jaw plate in place, and then tighten the bolts step by step in the order of "symmetrical cross". First, use your hands to preliminarily tighten all bolts, and then use a torque wrench to gradually apply force in 2-3 stages, tightening each stage in diagonal order. This can ensure even force distribution on the jaw plate and avoid deviation caused by improper tightening sequence.

Torque control: The tightening torque should be executed according to the requirements of the equipment manual, and should not be arbitrarily too large or too small. Excessive torque may cause damage to jaw plates or bolts, while insufficient torque cannot guarantee sufficient tightening force. When the equipment manual does not provide a clear torque value, the appropriate torque range can be selected based on the equipment specifications and bolt grades.

Anti loosening measures: The jaw plate is subjected to continuous impact and vibration during operation, and the fixing bolts are prone to loosening. Simply relying on tightening before starting up is difficult to fundamentally solve this problem. Spring anti loosening devices (such as spring washers, disc spring washers, or specialized spring anti loosening and vibration reduction devices) can be installed on the fixed bolts to enhance the anti loosening and vibration reduction capabilities and extend the service life of the jaw plate. The spring anti loosening device usually consists of an inner spring cover, a spring, and an outer spring cover. After tightening the nut on the bolt, the elastic force generated by the compression of the spring can effectively counteract the tendency of bolt loosening caused by crushing force.

3.4 Parallelism calibration and tooth profile matching

After installing the movable jaw plate, it is necessary to ensure that it maintains appropriate parallelism with the fixed jaw plate to avoid relative sliding between the jaw plate and the body during operation. Thin shims such as copper, lead, or zinc can be used to fine tune parallelism.

At the same time, the tooth profile of the movable jaw plate and the fixed jaw plate should be checked to ensure that the tooth peaks of the movable jaw plate align with the tooth grooves of the fixed jaw plate, so that the two are in a basic meshing state. This coordination method can subject the material to the dual effects of compression and bending during the crushing process, which is beneficial for improving crushing efficiency and also helps to reduce abnormal wear of the jaw plate.

3.5 Inspection and trial operation after installation

After installation, the following checks should be performed:

Check whether all fixing bolts have been tightened to the specified torque and whether the anti loosening device is installed in place.

Check for any obvious gaps or looseness between the jaw plate and the body.

Conduct a no-load test run to observe whether the equipment runs smoothly and whether there are any abnormal vibrations or noises.

After a period of trial operation, check the tightening status of the bolts again and re tighten them if necessary.

4、 Common Misconceptions and Precautions

Misconception 1: The thicker the cushion material, the better. The main function of cushion material is to fill small gaps and adjust parallelism, not the thicker the better. An excessively thick cushion layer may cause excessive deformation of the jaw plate after fastening, which in turn affects the contact effect. The appropriate thickness of cushion material should be selected based on the actual gap.

Misconception 2: Tighten the bolt completely at once. This operation can easily cause the jaw plate to deflect and experience uneven force, and a symmetrical step-by-step fastening method should be used.

Misconception 3: Neglecting the cleaning of contact surfaces. The residual material particles on the contact surface may seem insignificant, but they can become the starting point of local stress concentration, accelerating the local wear of the jaw plate and even causing fracture.

Misconception 4: Relying solely on tightening bolts before starting up to solve loosening problems. Jaw plates are subjected to continuous impact and vibration during operation, and bolt loosening is difficult to completely avoid. Structural measures such as installing anti loosening devices should be taken to solve the problem from the source.