Overspeed safety device

An overspeed safety device for a pneumatic rotation motor comprises a housing (10, 18) and a valve element (28) which is located in a bore (27) in the housing (10, 18) and which by a trip device (29, 31) is locked in a normally open position and arranged to block at least the main part of the air supply to the motor when shifted to a closed position, and an activating device (37, 38) coupled to a rotating part (14) of the motor and arranged to be shifted by inertia forces from a normal rest position to an active position at a predetermined motor speed level. The trip device (29, 31) comprises an open spring wire lock ring (29) pretensioned toward a closed condition and located in a groove (30) in the valve element (28), and a holding device (31) normally located between the ends of the lock ring (29) to maintain the lock ring (29) in an expanded condition in which its outer diameter exceeds the inner diameter of a shoulder (32) in the bore (27). At overspeed, the activating device (37, 38) moves the holding device (31) out of its lock ring (29) expanding position, thereby enabling the latter to contract and release the valve element (28) for movement toward a closed position.

BACKGROUND OF THE INVENTION 
This invention pertains to an overspeed safety device, particularly to an 
overspeed safety device for a pneumatic rotation motor, comprising a valve 
element guidingly supported in a bore in the motor housing and arranged to 
block at least partly the air supply to the motor when shifted from a 
normally open position to a closed position. Such device also comprises an 
activating means coupled to a rotating part of the motor and arranged to 
be shifted by inertia forces from a normal rest position to an active 
position at a predetermined motor speed level, and a trip means supporting 
the valve element in the open position and arranged to be triggered by the 
activating means. 
The main object of the invention is to create an overspeed safety device 
for a pneumatic motor that is simple and compact in structure yet reliable 
in operation. 
One example of previously known overspeed safety devices of the above type 
is described in U.S. Pat. No. 2,977,931. In this prior art device a 
cylindrical valve element is rotatably supported in a bore in the motor 
housing to control an air inlet passage, and an activating means is 
arranged to engage and move the valve element to closed position at a 
predetermined speed level. 
A drawback inherent in this known device is the large contact surfaces 
between the valve element and the housing necessary to guide a rotative 
element. These surfaces tend to increase the frictional resistance between 
the valve element and the housing, especially after a long inactive period 
when corrosive and/or other influence upon the surfaces has taken place. 
Another example is shown in G.B. Patent No. 1,366,482 where a valve spindle 
is arranged to be moved axially by a spring to block an axial air inlet 
passage when released by a trip means at activation of a fly-weight on the 
motor rotor. 
This known device suffer from the drawback of being able to control just a 
small inlet area, because an increased area would require a stronger 
spring to accomplish a closing movement of the valve element against the 
inlet pressure. A heavier spring load would in turn have a negative 
influence upon the action of the trip means, or would even make the use of 
a trip means impossible. 
The present invention intends to create an improved overspeed safety device 
by which the above problems are avoided. 
A preferred embodiment of the invention is described below in detail with 
reference to the accompanying drawings.

DETAILED DESCRIPTION 
In the drawing figures, an overspeed safety device according to the 
invention is shown in a grinding tool application. 
The illustrated tool comprises a housing 10 with a handle 11, an air inlet 
passage 12, a speed governor valve unit 13 and a rotation motor (not 
shown). In this particular embodiment of the invention, an air turbine is 
used as rotation motor, and the output shaft 14 of the tool is drivingly 
coupled to the air turbine via a reduction gearing (not shown). 
The speed governor valve unit 13 comprises a cylindrical valve member 16 
which is movably guided in a bore 17 in a valve housing insert 18. The 
valve member 16 is formed with a skirt portion 19 and radial openings 20. 
The latters coincide in the open position of the valve member 16 with an 
annular chamber 21 which communicates with the air turbine. 
In another larger bore 23 in the valve housing insert 18 there is movably 
guided a control piston 24 acting directly on the valve member 16. This 
control piston 24 is activated in one direction by a speed responsive 
control pressure derived from a pressure sensing means in the air turbine 
and supplied through a passage 25. In the opposite direction, the piston 
24 is acted upon by a spring 26 as well as by the air inlet pressure 
acting on the valve member 16. 
The overspeed safety device comprises an annular valve element 28 which 
surrounds the skirt portion 19 of the valve member 16 and which in its 
normal open position defines the annular chamber 21 together with the 
valve housing insert 18. See FIG. 1. 
The valve element 28 is located in a bore 27 in the valve housing insert 18 
and is supported in its normal open position by a trip means formed by a 
spring wire lock ring 29 partly received in a circumferential groove 30 on 
the valve element 28. See FIG. 4. The lock ring 29 is open and made 
maintain an expanded condition by a holding means in the form of a lever 
31 located between the ends of the lock ring 29. See FIG. 3. The lock ring 
29 is pretensioned toward closed condition. 
In its expanded condition, the lock ring 29 has an outer diameter that 
exceeds the inner diameter of an annular shoulder 32 in the valve housing 
insert 18, and, thereby, forms an axial lock means for the valve element 
28. See FIG. 4. A spring 33 exerts an axial bias load on the valve element 
28. 
The lever 31 is pivotably supported at its one end on an O-ring 35, and for 
fixing the lever 31 longitudinally it is formed with an indentation to 
engage the 0-ring 35. At its opposite end, the lever 31 is acted upon by a 
compression spring 36, the purpose of which is to bias the lever 31 
towards its lock ring 29 expanding position. See FIG. 1. 
An activating means in the form of a push rod 37 and a fly-weight 38 
mounted on the rear end of the output shaft 14 is intended to release the 
lock ring 29 by shifting the lever 31 against the action of the spring 36 
at a predetermined rotation speed. The fly-weight 38 is annular in shape 
and has its centre of gravity disposed off the rotation axis of the output 
shaft 14. The fly-weight 38 is formed with a radial, inwardly directed 
stud 40 which is received in a diametrical bore 42 in the shaft 14. A 
spring 43 located in the bore 42 acts on the stud 40 to exert a radial 
bias force on the fly-weight 38 so as to maintain the latter in its 
non-activating position at rotation speed levels below the predetermined 
intended release level of the safety device. 
Under normal operating conditions, i.e. when the speed governor operates 
correctly, the rotation speed of shaft 14 does not reach the level where 
the inertia force acting on the fly-weight 38 exceeds the bias force of 
the spring 43. This means that the push rod 37, the lever 31 and the lock 
ring 29 remain in their normal, inactive positions, and the valve element 
28 is supported in its open position. See FIG. 1. 
Should, however, the speed governor for some reason malfunction and not be 
able to limit the rotation speed as intended, the speed will rapidly reach 
the level where the inertia force acting on the fly-weight 38 exceeds the 
bias force of the spring 43. Then, the fly-weight 38 is displaced radially 
to hit the push rod 37 which in turn moves the lever 31 against the action 
of the spring 36. Thereat, the lever 31 is moved clear of the ends of the 
lock ring 29, see FIG. 2, which enables the latter to gain its closed, 
contracted condition. This means that the lock ring 29 retracts into the 
groove 30 and assumes an outer diameter that is smaller than the inner 
diameter of the shoulder 32. The valve element 28 is now free to be 
displaced toward its closed position by the action of the spring 33 and by 
the air pressure in the inlet passage 12. See FIG. 2. 
In its closed position, the valve element 28 covers the radial openings 20 
of the speed governor valve member 16, thereby preventing motive pressure 
air to pass the speed governor valve unit 13 and reach the air turbine. 
To reset the overspeed safety device, it is necessary to dismantle the 
entire mechanism. The trip mechanism is reset by expanding the lock ring 
29 by force and place the lever 31 in between the ends thereof to maintain 
the lock ring 29 in its expanded condition. 
The device described above and shown in the drawing figures is applied on 
an air turbine driven tool. In this application it is easier to obtain a 
reliable operation of the fly-weight activating means when locating it to 
the output shaft which is not the rotor of the motor. The very high 
rotating speed of the turbine rotor would create practical problems for 
the operation of a fly-weight actuator. 
However, it is to be noted that the invention is not at all restricted in 
its applicability to turbine driven tools, but could as well find its use 
at vane motor driven tools. In such applications, the fly-weight actuator 
is mounted on the motor rotor. 
The overspeed safety device according to the invention is advantageous in 
that it is simple and compact in design yet reliable in operation.