Rotary shaft brake

Details Rotary shaft brake Rotary shaft brake

2002-05-16

2004-09-07

Williams, Thomas (Department: 3683)

Brakes

Wheel

One-way brakes

C188S072100

Reexamination Certificate

active

06786309

ABSTRACT:

FILED OF THE INVENTION
The present invention relates to hydraulic or fluid-activated brake assemblies and, in particular, to safety brake assemblies for drive strings which store energy as reactive torque by reason of being under torsional strain.
BACKGROUND OF INVENTION
Certain drive systems are subject to torque and large torsion (rotary strain) due to their length. As a result, they store a large amount of energy. When drive power to the system is interrupted, the torsion is released as back-spin and, if an uncontrolled release of torsional energy occurs, personal injury and/or property damage can result. For example, deep well submersible pumps such as progressing cavity pumps (PCPs) driven by sucker rod strings are commonly used to pump oil from deep wells. The drive strings of these submersible pumps usually have a relatively small diameter, typically of ¾ to 1⅛ inches. Such drive strings are commonly used in wells that vary from 1,500′ to 6,000′ in depth, 3,000′ being a common average. Progressing cavity pumps include a stator which is attached to a production tubing at the bottom of a well and a rotor which is attached to a bottom end of the drive string. They are frequently used to pump viscous crude oil, often laden with sand or other impurities, which requires a large amount of torque. As a result, the elongated drive string is subject to considerable torsional strain. This torsional energy is stored in the elongated drive string as reactive torque. In a 3,000 foot string, as many as several hundreds of revolutions of torsion can be stored in the string if viscous sand laden crude oil is being pumped. When power is interrupted to the drive string, the torsional strain is released. Unless the release of torsion is controlled, costly and undesirable damage to equipment and/or personal injury to workmen in the vicinity can result. This is particularly true if an electric motor is used as a power source because such motors offer almost no resistance to reverse rotation when the power supply is interrupted.
Whenever drive power is interrupted to the drive string of a progressing cavity pump, the torsional strain is desirably released in a controlled fashion. Brakes which simply prevent the release of the torsion in the drive string are unsatisfactory for two reasons. First, it is preferable that in the case of an electric motor drive, the motor restart unattended when power is restored. In order to ensure a successful unattended restart, the motor must start without load. If the reactive torque is not released, the motor may not be capable of restarting and may be damaged as a result. Second, if pump repair or replacement is required any unreleased torsion in the drive string can be extremely dangerous for unaware workmen. Severe personal injury can result from the unintentional release of torsion in such drive strings.
Consequently, braking systems have been developed for control of the back-spin due to torsion in elongated drive strings. One system is a fluid brake that includes a pump engaged only when torsion is released from the drive string. The pump is used to circulate hydraulic fluid or lubricating oil from a reservoir through a restricted orifice. The resistance of the fluid created by the restriction serves to control the release of torsion. Nonetheless, this apparatus is subject to several disadvantages including excess heat production leading to potential lubricant breakdown, lack of adjustment to different load levels, and uneconomical size.
Other brake systems for controlling the release of torsion have been disclosed. Those brake systems are based on centrifugal braking principles. Examples of such braking systems are found in U.S. Pat. No. 4,216,848 which issued to Toyohisa Shiomdaira on Aug. 12, 1980; U.S. Pat. No. 4,797,075 which issued to Wallace L. Edwards et al on Jan. 10, 1989 and U.S. Pat. No. 4,993,276 which issued to Wallace L. Edwards on Feb. 19, 1991. The brakes disclosed in these patents are difficult to maintain due to internal mechanical components and require frequent maintenance when they are used to stop the reverse rotation of drive strings in a controlled manner. In addition, they are mechanically complicated and include custom-made moving parts which are costly to manufacture and expensive to keep in inventory. These brakes also tend to over-heat and burn out an oilwell progressing cavity pump system, because of the long brake cycle created by the fluid in the production tubing draining through the pump, which causes it to motor backwards and drive the drive string backwards for an extended period (up to 45 min).
A much improved brake system is known from U.S. Pat. No. 5,358,036 which issued to Robert A. R. Mills on Oct. 25, 1994. That patent discloses a relatively simple hydraulically operated disc brake mechanism which is self regulating and ensures that the torsional energy is completely dissipated in a controlled release. The brake mechanism permits axial rotation of a shaft in a first direction while retarding axial rotation of the shaft in an opposite direction. The brake mechanism includes a housing for the bearings of a downhole pump drive head which housing also functions as a reservoir for the lubricating oil for the bearings. The shaft drives a bi-directional hydraulic pump. When the shaft turns in an operating direction, the pump forces the lubricating fluid in a closed circuit through a filter and back to the reservoir. If normal operation is interrupted, and the torsion in the system is released, the bi-directional pump forces oil through a control manifold to a disc brake caliper which brakes a disc rotor mounted on the shaft. The faster the shaft turns as it releases torsion, the more braking force is applied. As the torsion is released, the reactive torque and the shaft speed drop and the brake force is correspondingly reduced until all torsion is released from the shaft, permitting safe handling and soft restart.
One problem encountered with this braking system is a build up of gas or vapour in the hydraulic circuit, especially the brake caliper of the disk brake, causing brake failure due to ‘air lock’. This is caused, in the first instance, by air entrainment in the lubricating oil in the bearing/gear box of the well head by agitation of the oil by the bearings, and by the gears in the case of a right angle drive. As oil is continuously drawn from the bearing/gear box during operation of the drivehead, for the purpose of lubricating the top bearing, aerated oil is being circulated continuously through the hydraulic circuit manifold. Air tends to separate from the oil and accumulate at locations with little or no flow, such as the brake lines and brake caliper wherein no oil flows during forward rotation of the drive string. Although periodic manual bleeding of the brake caliper and flushing of the brake system will largely prevent these problems in deep well PCP systems, manual intervention is expensive and completely unacceptable in the deep well pumping field, where the customer demands complete reliability without manual intervention throughout the lifetime of the pump.
In the second instance, another source of gas and/or vapour that is a common problem of disc brakes under severe duty is excessive heat generated during the braking cycle causing the hydraulic fluid to boil and release vapour. The developed gas will remain in the brake system and cause partial or complete brake failure, a condition known as vapour lock.
Thus, a disk brake system is required which overcomes these problems.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a relatively simple, reliable brake system which is easy to maintain, the system permitting axial rotation of a shaft in one direction while retarding axial rotation of the shaft in the opposite direction and for substantially preventing brake fade caused by the accumulation of gas and or vapour in the hydraulic system.
It is a further object of the invention to provide a disc brake system for controllably releasing torsion stor

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