Ball-jointed pin-key universal shaft‌

At present, the main structures of universal shafts for positive displacement motors (PDMs) include petal-type, ball-jointed type and titanium alloy universal shafts. The petal-type universal shaft is mainly applied to PDMs with fixed bend angle structures, while the ball-jointed universal shaft and titanium alloy universal shaft are mostly used in PDMs with adjustable bend angle structures.

The ordinary petal-type universal shaft emerged the earliest and is the most widely used at present. However, restricted by its structure and process, its service life is at an average level, and its performance has been developed to the extreme with no room for further improvement.

The titanium alloy universal shaft realizes the universal conversion function relying on the high specific strength and good flexibility of titanium alloy. Although it has a relatively long service life, it has two major drawbacks: first, it is extremely expensive, and the supply of this material is monopolized by state-owned military enterprises, leading to unstable supply and difficult procurement; second, its heat treatment requirements are very complex. Improper treatment will cause early failure and rapid fracture, which makes enterprises get no benefit but suffer great risks.

The ball-jointed universal shaft is a new type of universal shaft developed in recent years. It can be used in adjustable bend housings and can also replace the petal-type one, boasting a wide application range. Compared with the ordinary petal-type one, its most prominent feature is the sealed lubrication structure, which enables the motion and force transmission components to work in an oil bath environment without being scoured by mud, thus prolonging the service life.

Currently, it has the following structural forms:
(1) Ball-tooth type
(2) Spline type
(3) Ball-rod type

(1) Ball-tooth universal shaft
The ball-tooth universal shaft realizes universal swing connection through ball hinge, and transmits torque through steel balls, so as to transfer the planetary motion and torque of the rotor to the transmission shaft and realize the drilling of the PDM. This universal shaft has a simple structure and flexible operation. Its disadvantage lies in the small contact area between the steel ball and the ball socket of the intermediate shaft as well as the inner cavity groove of the sub. The local contact stress is too large, resulting in severe wear and early failure. (Domestic raw materials and heat treatment processes still lag behind those of Western developed countries.)

(2) Spline universal shaft
The spline universal shaft is developed to solve the problem of too small contact area between the ball tooth, the ball socket and the groove. It uses internal and external splines to transmit torque, which increases the contact area to a certain extent. However, due to the planetary motion of the rotor and the centering motion of the transmission shaft, the internal and external splines cannot fit together stably and tightly to transmit torque, and interference occurs during the movement, causing additional destructive stress.

(3) Ball-rod universal shaft
The ball-rod universal shaft is a variant of the ball-tooth type. It has a hemisphere in the middle of the semi-cylinder, which is installed in the ball socket of the intermediate shaft. The semi-cylinder is inserted into the inner cavity groove of the sub as a force transmission component. The ball-rod only unilaterally increases its contact area with the groove of the sub, while the contact area with the ball socket of the intermediate shaft remains unchanged. The hemisphere and the ball socket are still its weak links.

On the basis of fully studying the characteristics of the above several ball-jointed universal shafts, our company has made innovations by replacing the ball-tooth transmission with pin-key transmission, thus inventing a ball-jointed pin-key universal shaft.

We use the pin-key as the component for transmitting torque. One end is made into a cylindrical pin, and the other end is made into a key. A cylindrical hole is machined on the outer circle at the center of the ball hinge of the intermediate shaft, and the cylindrical end of the pin-key is inserted into the hole, so that the pin-key can rotate freely in the hole. At the same time, a keyway is machined on the wall of the sub hole. After the intermediate shaft is assembled with the pin-key and inserted into it, the intermediate shaft can freely swing around the center of the ball hinge in the sub, and at the same time, it fits with the pin-key and the keyway without interference to transmit torque stably.

The advantages of the pin-key universal shaft are as follows:

1. Simple structure, easy for processing and assembly. The contact area between the pin-key and the keyway on the inner cavity of the sub is large. Meanwhile, the pin-key cylinder is matched with the round hole of the intermediate shaft and can rotate freely, which eliminates additional stress during operation, enlarges the contact area, and endows the shaft with strong wear resistance and extrusion resistance, thus realizing large torque transmission.

2. The main transmission parts of the pin-key universal shaft are made of high-strength alloy steel with excellent performance. Through optimized structural design and surface hardening treatment, the parts truly achieve a hard surface and tough core, enabling the transmission components to withstand high stress while having excellent wear resistance.