Variable torque rate simulated test joint

Details Variable torque rate simulated test joint Variable torque rate simulated test joint

1999-03-03

2001-03-13

Hoff, Marc S. (Department: 2857)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Mechanical measurement system

C702S041000, C702S042000, C073S862080, C073S862230

Reexamination Certificate

active

06202028

ABSTRACT:

DESCRIPTION
1. Field of the Invention
The invention relates to calibration equipment for testing the accuracy and consistency of rotary power assembly tools for threaded fasteners, and provides a variable torque rate simulated test joint against which such rotary power assembly tools can be tested.
2. Background Art
Rotary power assembly tools for the application of torque to screw-threaded fasteners can include power screwdrivers, torque wrenches, pneumatic nutrunners and hydraulic impulse tools. Power screwdrivers tend to be used for the more lightweight applications where the torque to be applied is only a few N.m (Newton.Meters), whereas torque wrenches and hydraulic impulse tools are used to deliver torques of up to 150 N.m or even above. Hydraulic impulse tools are becoming progressively more widely used for production line assembly work, since they are torque-absorbing in the sense that they do not require the operator to exert a significant counter-torque to prevent tool movement, and are generally faster and more convenient to use than torque wrenches. There is however no calibration equipment commercially available against which all such tools can periodically be checked to ensure consistency of torque application in the resulting joints.
In an assembly production line, for example, a worker may be using the same rotary power assembly tool for tightening large numbers of threaded bolts to a predefined torque. That predefined torque is set into the rotary power assembly tool but the setting could vary over time as the tool is subjected to heavy use. A simulated test joint is therefore provided, against which the rotary power assembly tool can be tested periodically to ensure that it is not in need of maintenance or adjustment. Generally the test joint is designed to mimic in a known repeatable way the behaviour of an idealized real joint. This allows the performance of the tool to be gauged accurately.
Real joints do not all have the same “Torque Rate”. “Torque Rate” is defined in ISO 5393 as the “increase in torque with angular displacement while advancing a fastener in a threaded joint”. In other words, if an increase in applied torque of &Dgr;&tgr; results in an increase in the angular displacement of a threaded fastener of &Dgr;&agr;, torque rate is the ratio &Dgr;&tgr;/&Dgr;&agr;. The threaded joint may be a hard joint or a soft joint, of which hard joints have the higher torque-rates. Consequently, in order to test a power tool in circumstances which mimic those encountered in reality, test joints are available which have variable torque settings and variable torque rate settings. ISO 5393 is the International Standard relating to test joints and it defines, for non-impact wrenches, specifications for the &Dgr;&tgr;/&Dgr;&agr; relationship for hard (high [H] torque-rate) and soft (low [L] torque-rate) joints. These specifications are intended to represent the two ends of the range of hard and soft joints encountered in normal usage of a power tool.
DE-C-3305457 discloses a variable torque rate simulated test joint for a rotary power assembly tool which applies torque continuously. The tool is used to apply torque to a shaft which rotates against a magnetic brake. The braking force is increased until the shaft stops. Torque and angle are monitored repeatedly or continuously during braking. A microprocessor compares the torque and angle values and controls the braking force as a function of the measured angle, such that the ratio of torque to angle has a specifiable constant value. The use of an angle feedback ensures that the simulated test joint can comply with ISO 5393.
The simulated test joint of DE-C-3305457 is not suitable for use with hydraulic impulse tools or with any other pulse or impact operated power assembly tool.
GB-A-2038006 discloses an apparatus for checking and adjusting a power screwdriver to a predetermined tightening torque. The apparatus comprises a purpose-designed simulated joint specific to the screwdriver to be adjusted, and comprises a disc brake straddled by brake calipers of the kind used in motor car braking systems. The choice and design of components in GB-A-2038006 is such as to teach the reader that the inertia of the components is immaterial. High inertia components are used in the illustrated preferred embodiment, and there is no appreciation in the teaching of GB-A-2038006 that the device may be capable of broader utilization if the basic design elements of the apparatus were discarded and the apparatus redesigned to achieve minimum inertia. Indeed there is no appreciation that a low inertia friction brake achieving the necessary torque levels is achievable, or that it would be useful.
In a hydraulic impulse tool a flywheel is accelerated to build up its energy store over a very short period, of the order of 10 to 30 milliseconds, during which period no output torque is delivered. There is then a momentary transfer of that accumulated energy, as an impulse, to the joint or simulated joint. During that transfer the output torque delivered rises from zero to its maximum value and falls back again to zero over a very short space of time, typically from 1 to 10 milliseconds. Moreover during use of a hydraulic impulse tool the angular movement of the joint components per impulse of the tool varies, and the rate of pulsing of the tool also varies. Thus no monitoring of torque rate and angle as in DE-C-3305457 could possibly give rise to a meaningful or sufficiently fast and responsive control of brake application in a simulated test joint used in conjunction with a hydraulic impulse tool. There is no way at all that an apparatus according to GB-A-2038006 could have any significant use with a hydraulic impulse tool.
Further, it has been realized by the Applicants that the apparatus of DE-C-3305457 could not possibly be used as a test joint for a pulse tool even if used in a different way to that claimed in DE-C-3305457. DE-C-3305457 applies its variable torque by means of an electromagnetic brake which brakes a shaft coupled to the nutrunner of the device. The inventors of this apparatus considered it to have a low inertia and indeed when testing non-pulse tools its inertia can be regarded as virtually negligible. However, the Applicants have discovered that inertia is extremely significant when testing pulse tools, as discussed later. The inertia of the apparatus of DE-C-3305457 would be high enough to make it totally unsuitable for the purpose. The inertia of the apparatus of GB-A-2038006 is several orders of magnitude higher still.
U.S. Pat. No. 4150559 discloses a variable rate joint suitable for calibration of a power angle wrench or nutrunner, in which the joint rate can be selected between a hard joint and a soft joint by actuating one or other of a pair of electromagretic valves to direct pressurized control air to a disc brake mechanism. The valves direct the air through alternative passages one of which incorporates a flow restrictor to slow down the brake actuation for the sofe joint simulation. The apparatus is totally unsuitable for use with impulse tools which are not even considered in U.S. Pat. No. 4150559.
THE INVENTION
The invention provides a variable rate test joint comprising:
a housing;
a shaft mounted within the housing;
means for coupling a tool to be tested to the shaft; and
brake means for applying a braking torque to the shaft;
characterized in that
the brake means comprises
a brake shoe assembly actuable by electrohydraulic or electropneumatic means and being arranged to act in use directly on the outer cylindrical surface of the shaft to apply a frictional braking torque thereto; and
a computer for controlling the pressure applied to the electrohydraulic or electropneumatic means as a function of time, being arranged to vary in use the applied pressure from a preset threshold to a maximum value over a time period which is variable to reflect the hardness of the joint being simulated.
Because frictional means are used to brake the shaft, it can be of small outside diameter. It can also

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