Well Service Pump Crankshaft

A forged crankshaft is an important component in the frac pump power end which reverts the reciprocating motion of the plunger into rotational motion. This helps in achieving rotational power output from the reciprocating input given by the plunger. It works according to the upward and downward movement of the plunger. A forged crankshaft connects with the plunger through a connecting rod. The power end fluid end crankshaft bears the force transmitted from the connecting rod and then converts it into torque, then output to other accessories at the power end pump. The crankshaft parts are subjected to bending and torsion loads, which makes the crankshaft more subject to wear and needs to be replaced periodically.

The following fig (Fig 1 ) is a crankshaft for quintuplex frac pump, which is an integral structure with six supports and five curved shape shafts. It is supported by 6 heavy-duty cylindrical roller bearings. It is a statically indeterminate structure with two-way positioning, good rigidity, and reliable operation. There are oil passages connected in each curved shaft to lubricate the connecting rod bearings. In crankshaft production, the forged crankshaft is made of alloy steel forgings, which need to be heat treated and polished.

The motor gear drive drives the crankshaft to rotate. Through the crank slider mechanism, the rotary motion of the original moving part is converted into the reciprocating linear motion of the crosshead in the slideway, so as to perform suction and discharge work.

The main load that the pressure pump receives under normal operation is provided by the power end and the hydraulic end, where the power end load is constant, and the hydraulic end load is. As the crankshaft rotates and changes, the key components are mainly loaded by the hydraulic end.

As we said crankshaft manufacturers should assure the crankshaft manufacturing soundly because the crankshaft parts experience a large number of load cycles during their service life.

Welding crankshaft always has a problem during manufacturing, the weld of the cams would change the large diameter. This would create stress concentration problems.

Crankshaft manufacturers always take casting crankshaft as a good choice, casting crankshaft production is strong and sometimes quite reliable. However,  crankshaft manufacturers can’t ignore the casting anomalies which are undetectable in inspection, like porosity and inclusions.

Welding or casting crankshaft production is arguable. A forged crankshaft is one way of avoiding all the above potential problems. The forged crankshaft is unlikely to fatigue failure under normal working condition

When selecting the material for frac pump power end crankshaft manufacturing, it should have sufficient stress strength limit and wear resistance. Typically, AISI 4140 is widely used in fracturing crankshaft manufacturing. It has combined various favorable factors after integrally forged, the power end crankshaft parts still need heat treatment and machined to make the surface of the crankshaft parts reach the required accuracy.

42CrMo/AISI4140 Steel Chemical Composition

The fatigue failure of a part refers to the failure of the material under repeated and relatively low alternating stress. Normally, fatigue failure is generally manifested as a sudden crack. There is no obvious deformation or other manifestation before fatigue failure happened. It is difficult to find out in advance during use. Therefore, the damage of fatigue failure is critical.

The stress when fatigue failure occurs is much smaller than the strength limit or even the yield limit of the material. The fatigue failure of parts can be divided into the following four stages:

• The accumulation of plastic deformation of parts
• The generation of initial cracks
• The expansion of fatigue cracks
• Fatigue fracture.

It usually depends on the shape and size, material characteristics, use conditions, surface conditions, external environment, and other factors. Therefore, the fatigue failure of a part is a complex failure process affected by many factors.

With the increasing horsepower of the fracturing pump, the problem of crankshaft fatigue failure has become more prominent. The crankshaft parts are subjected to extreme bending loads and stress concentration areas.

The forged crankshaft has a complex geometric shape and bears huge alternating loads under fracturing conditions. Due to its very irregular structure and cross-section shape change a lot, the stress concentration is serious, and fatigue failure is prone to occur.

According to practical experience, 90% of failures are caused by fatigue failure due to material damage or deterioration of parts. The fatigue life of a forged crankshaft is closely related to its material properties, working load, stress concentration factor, and the quality of the machined surface.

The frac pump power end forged crankshaft is subjected to the combined action of the bending and torsion of the connecting rod, the most vulnerable parts are the crank pin journal oil hole, the transition fillet between the inner surface of the crank arm, and the main journal and the key-way connection section.

There is an estimated working life span of frac pump power end crankshaft production, which is analyzed under water hammer fatigue experiment, are 500 hours. Most of the damage to the crankshaft of the high-power fracturing pump is caused by torsional fatigue. In order to ensure the power end crankshaft can work reliable and meet the service life requirements, it is necessary to check the fatigue strength by the crankshaft manufacturer.

Please keep the frac pump running under the rated pressure, leave a safe pressure margin Don’t let the crankshaft parts be subjected to extended maximum load.

We suggest you should allocate the crankshaft parts working time reasonably, and conduct a comprehensive inspection or replace crankshaft parts every 500 hours. And it is important to place reinforcing ribs or sealing plated at the bearing seat to reduce or avoid harmful vibration.

TianyuMfg., as a forged crankshaft manufacturer, chooses better materials and manufactures the power end crankshaft parts with the surface be treated with stress relief, no sharps, burs, scratches, and other defects, to ensure its safety and reliability.