Telescopic piston

Details Telescopic piston Telescopic piston

1999-03-02

2001-05-22

Ryznic, John E. (Department: 3745)

Expansible chamber devices

With releasable stop or latch means to prevent movement of...

Means includes element interfitting between working member...

C092S053000

Reexamination Certificate

active

06234062

ABSTRACT:

This invention relates to fluid actuators and particularly although not exclusively pneumatic or gas powered actuators. High speed actuators are often energised by compressed gas when high forces and speed of actuation are vital, such as in emergency release, ejection or actuation systems.
A major problem with such systems which use a discrete volume of pressurised gas, is the large gas storage receiver needed to maintain a reasonably sustained pressure as the internal swept volume of the actuator increases during its stroke. The measures of effectiveness are the energy efficiency and thrust efficiency of the system, and the velocity imparted to an inertial load.
In the following examples, the maximum permitted force is assumed to be 22 kN, the inertial load (ejection mass) a mass of 153 kg and the gas reservoir is 500 ml at an initial pressure of 20 MPa. These might be typical values for an ejector ram pressurised by compressed air or nitrogen.
Under these conditions a typical approximated force/stroke diagram for a single stage 20 compact ram such as is schematically shown in
FIG. 1
a
appears in
FIG. 1
b
. The thrust efficiency, expressed as the equivalent average force divided by the peak force, is summarised in Table 1, together with the energy efficiency expressed as the expansion work done by the gas divided by the total energy available from adiabatic expansion of the gas to zero relative pressure. The importance of the peak force is that it is usually limited by the physical properties of the item being ejected or the allowable reaction force which can be tolerated by the launch platform Energy efficiency is important in achieving a high ejection mass final velocity from a given volume of compressed gas.
TABLE 1
Work done first portion (i.e. 0-approx. 150 mm stroke) =
2676
j
Work done second portion (i.e. approx. 150-300 mm
1859
j
stroke) =
Total work done =
4535
j
Mass final velocity =
7.70
m/s
Average effective force =
15120
N
Peak force/Average effective force =
1.45
Peak acceleration =
144
m/s
2
Peak ‘g’ =
14.7
‘g’
Thrust efficiency =
68.7%
Energy efficiency =
22.7%
Greater energy (and hence final mass velocity) may be extracted from the gas by lengthening the cylinder and piston (
FIG. 2
a
), if space permits. The result is of the form shown in
FIG. 2
b
, and will be seen to exhibit degraded trust efficiency in exchange for the improved energy efficiency (Table 2). However, space is often at a premium in emergency release installations, and also, the slender ram which results, will be subjected to lateral forces at its end during extension, and for sufficient robustness will need to be excessively heavy.
Therefore, this option is generally not used.
TABLE 2
Work done first portion =
4428
j
Work done second portion =
2466
j
Total work done =
6894
j
Mass final velocity =
9.49
m/s
Average effective force =
11890
N
Peak force/Average effective force =
1.85
Peak acceleration =
144
m/s
2
Peak ‘g’ =
14.7
‘g’
Thrust efficiency =
54.0%
Energy efficiency =
34.5%
Telescopic piston assemblies are used to obtain greater ram stroke, and hence energy output, from a given actuator installed length. In their simplest form as shown in
FIG. 3
a
, their lateral stiffness is good because, if the sequence of extension is unrestrained, the high initial gas pressure acts on the largest piston area first, and as the gas expands, its reduced pressure then acts on the smallest area. But for the same reasons the thrust and energy efficiencies are poor. Nonetheless, a modest increase in energy output/installed length is obtained. The results are of the general form shown in
FIG. 3
b
and Table 3.
TABLE 3
Work done first stage =
4535
j
Work done second stage =
1051
j
Total work done =
5586
j
Mass final velocity =
8.54
m/s
Average effective force =
9630
N
Peak force/Average effective force =
2.28
Peak acceleration =
144
m/s
2
Peak ‘g’ =
14.7
‘g’
Thrust efficiency =
43.8%
Energy efficiency =
27.9%
Ejector rams have been designed, especially for use with ‘hot gas’ (i.e. as generated by a pyrotechnic gas generator or ‘cartridge’), to ensure that the highest pressure acts upon the smallest area first, (see UK patent GB 2 078 912 B and
FIG. 4
a
herein) but even this is an incomplete solution because eventually, the volume masked from the high pressure gas during the first stage of ram extension is suddenly exposed to the gas, and the resultant expansion and depressurisation negates much of the advantage of having a larger working area during the second stage. Again, a further modest improvement in energy output is obtained, but the resultant force/stroke characteristic is still far from ideal, and is shown in
FIG. 4
b
and Table 4.
TABLE 4
Work done first stage =
4535
j
Work done second stage =
1767
j
Total work done =
6302
j
Mass final velocity =
9.08
m/s
Average effective force =
10870
N
Peak force/Average effective force =
2.02
Peak acceleration =
144
m/s
2
Peak ‘g’ =
14.7
‘g’
Thrust efficiency =
49.4%
Energy efficiency =
31.5%
U.S. Pat. No. 4,850,553 (E. K. Takata et al, see
FIG. 5
a
herein) proposes a telescopic piston which extends in a desirable manner, giving an improvement in efficiencies, but which still has two deficiencies:
a) It is structurally complex and, because the slenderest ram component extends first, does not maximise lateral stiffness.
b) The internal volume of the ram is still excessive because the smaller piston is filled with gas before any useful work is done by the ram.
An estimate of the performance characteristics of such a device is shown at
FIG. 5
b
and Table 5.
TABLE 5
Work done first stage =
3546
j
Work done second stage =
3525
j
Total work done =
7071
j
Mass final velocity =
9.61
m/s
Average effective force =
12190
N
Peak force/Average effective force =
1.51
Peak acceleration =
120
m/s
2
Peak ‘g’ =
12.3
‘g’
Thrust efficiency =
66.2%
Energy efficiency =
35.4%
The Invention
The invention provides a piston assembly comprising an inner component, an intermediate component and an outer component, all telescopingly interfitted together, the inner component comprising a fluid outlet at one end, the intermediate component making a sliding seal with the inner component and comprising a closed end surrounding the fluid outlet end, the outer component making a first sliding seal with the intermediate component and a second sliding seal with the inner component, and a detent operative to hold the outer component in an extended position relative to the inner component.
This arrangement can offer a reduced size gas storage volume, and/or a significant improvement in energy efficiency compared with conventional art, by providing a more sustained thrust from the extending ‘ram’ in a manner which will be described hereunder. This may be achieved without compromise to the structural efficiency of the ram assembly under the influence of lateral forces during extension.


REFERENCES:
patent: 668321 (1901-02-01), Sonnex
patent: 1799298 (1931-04-01), Jakob
patent: 3010752 (1961-11-01), Geffner
patent: 3186308 (1965-06-01), Butterworth
patent: 3426651 (1969-02-01), Arendarski
patent: 3614912 (1971-10-01), Nepp
patent: 3958376 (1976-05-01), Campbell
patent: 4075929 (1978-02-01), Peterson
patent: 4088287 (1978-05-01), Hasquenoph et al.
patent: 4388853 (1983-06-01), Griffin et al.
patent: 4466334 (1984-08-01), Holtrop
patent: 4511117 (1985-04-01), Soderstrom
patent: 4850553 (1989-07-01), Takata et al.
patent: 5850713 (1998-12-01), Hojo
patent: 6073886 (2000-06-01), Jakubowski, Jr. et al.
patent: 2 078 912 (1983-02-01), None

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