Double-acting hydraulic cylinder having two pistons arranged for coaxial movement relative thereto
BACKGROUND OF THE INVENTION 
     In existing hydraulic cylinders intended for telescopically displaceable crane arms and comprising two pistons which are movable coaxially relative to their cylinder, the end surface of the larger piston has a size which is equal to the inner end surface of the cylinder jacket. The return side of the piston, on the other hand, has a considerably smaller operative surface. When a hydraulic cylinder of this kind is used to increase or reduce the range of a telescopically movable crane boom, it is necessary that each piston moves at essentially the same speed, both in the outwards and the inwards directions. This requirement cannot be satisfied, as the above-mentioned large difference in piston surfaces at the front and rear sides of the piston means that if the fluid volumes are equal, the piston is imparted a considerably higher speed in the inwards direction than in the outwards direction. As a consequence of this increase of speed, a large volume of return oil must likewise be forced out of the cylinder in a limited period of time, which means that the flow velocity in the conduits increases to critical values, unless the conduits are dimensioned to accommodate the larger fluid quantities. However, it is expensive to have to oversize tubes and ipe conduits. 
     Upon rapid piston movements outwards, such as is the case when the crane boom is directed downwards and the load is heavy, one consequence of the large piston surface differential may be that there is not enough time to allow a sufficient quantity of oil to refill the piston return side in the cylinder, and therefore a vacuum may generate in this area, causing the piston, during its return movements, to strike back across the &#34;void&#34; in an uncontrolled manner. This constitutes an element of risk to the crane operator and anybody in the immediate vicinity of the crane. In addition, cavitation damages may occur. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention which relates to hydraulic cylinders of the type mentioned, is primarily to remedy the deficiencies outlined above. More precisely the invention concerns a double-acting hydraulic cylinder having two concentrically arranged jackets forming between them an annular chamber at the ends of which are provided pressure-fluid inlet and outlet ports and in which chamber is axially movable an annular piston having a tubular piston rod which is arranged for axial displacement in a sealed annular gap formed between one of the end sections of the annular chamber and the inner jacket, wherein a second piston is arranged for axial movement. 
     Another purpose of the subject invention is to ensure, in a double-acting hydraulic cylinder of this type intended to drive e.g. a telescopically movable arm, that each one of the two pistons, the movements of which are independent of one another, is displaced at essentially the same speed when moving outwards and when moving inwards, while at the same time is ensured that the pistons are displaced in the correct order, i.e. that when the telescopic arm is moved outwards, the outer piston is first displaced outwards and then the inner piston, while when the telescopic arm is moved inwards, first the inner piston and then the outer piston are displaced inwards. This effect is obtained in accordance with the present invention in that the inner jacket is provided with a bottom section and that the opposite one of the end sections of the cylinder is provided with a pressure medium inlet and outlet port adjacent the inner jacket end positioned in this area, that this jacket is provided adjacent said bottom section thereof with openings through which the end of the annular chamber that is positioned between said opposite end section and the annular piston, communicates with the space of the inner jacket that is located between the bottom section and the inner piston, and in that between the annular piston and the inner jacket is formed an annular gap which communicates with said openings. 
     Owing to this arrangement, the relative speeds of the outer piston, i.e. the annular piston, and the cylinder are approximately equal during outwards and inwards movements. In addition, the pressurized oil supplied to the cylinder and the return oil withdrawn therefrom will flow at essentially the same speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional characteristics of the invention will appear from the following description with reference to the accompanying drawings, wherein 
     FIG. 1 illustrates, partly in an axial longitudinal sectional view, a broken side view of a double-acting hydraulic cylinder in accordance with the invention, comprising two pistons which are displaceable coaxially relative to the cylinder. 
     FIGS. 2 and 3 illustrate on a slightly smaller scale the same views but with the cylinder moved to the far right on the stationary, larger piston and the smaller piston displaced to the right in the cylinder. 
     FIG. 4 is a side view of a crane incorporating the hydraulic cylinder in accordance with the subject invention positioned on the telescopic arm of the crane, and 
     FIG. 5 is a similar view showing the hydraulic cylinder and the pistons thereof in the positions corresponding to those shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     The hydraulic cylinder 1 illustrated in the drawings comprises an outer jacket 2 and an inner jcket 3 arranged coaxially with the inner jacket but having an outer diameter which is smaller than the inner diameter of the outer jacket, whereby an annular chamber 4 is formed between the jackets 2 and 3. At one end of the outer jacket, the left end, is attached an end section 7 which is provided with sealing rings 5, 6, and at the opposite end, between the outer jacket 2 and the inner jacket 3, is disposed an end section 10 provided with sealing rings 8 and 9. At the ends of the outer jacket 2 are arranged ports 11, 12 for outlet and inlet of a pressurized medium, in this case pressurized oil. An annular piston 13 is arranged for axial displacement in the annular chamber 4 and is provided with a tubular piston rod 14 which is displaceable in an annular gap 15 formed between the end section 7 and a bottom section 17 of the inner jacket 3, said bottom section provided with sealing rings 16. Inside this jacket is axially displaceable a piston 18 having a piston rod 19 which passes through an end piece 22 provided with sealing rings 20 and with an outlet and inlet port 21 for pressurized medium. The piston rod 14 has an inner diameter which is somewhat larger than the outer diameter of the inner jacket 3, whereby an annular gap 23 is formed between these two parts, and adjacent the bottom section 17, the inner jacket 3 is formed with openings 24 through which openings the space 25 formed between the bottom section 17 and the piston 18 inside the inner jacket 3 communicates with the annular chamber 4 and the annular gap 23. 
     The outer (left) end of the piston rod 14 which is closed by an end piece 26, is provided with a transverse shaft 27 securing the piston rod 14 to a fork-shaped attachment 28 (FIG. 4) on the rocker arm 29 of a loading crane (of which only the upper part is shown in FIGS. 4 and 5). The hydraulic cylinder 1 is provided with two transverse, coaxial pins 31 by means of which the cylinder may be attached to a fork-shaped attachment 32 on the outer end (the right-hand end in accordance with FIGS. 3 and 4) of a telescopic part 34 arranged for movement in the rocker arm 29 or in a tubular sleeve 33 secured on one side of said arm. The outer (right-hand) end 36 of the piston rod 19 may be secured to a head 37 provided on a telescopic part 38 which is axially displaceable in the telescopic part 34. In the head 37 is intended to be suspended the load to be lifted by the crane. 
     When the telescopic parts 34 and 38 are to be pushed out of the tubular sleeve 33 to increase the range of the crane, oil which is supplied to a (not illustrated) pressurized oil container by a pump, is forced through the inlet port 12 to the annular chamber 4 in the space 39 between the annular piston 13 and the end section 10. This forces the cylinder 1 to move to the right, bringing along its telescopic part 34, whereby return oil is forced from the annular chamber behind the annular piston 13, out through the outlet port 11. Only a portion of the quantity of oil forced through the inlet port 12 to the annular chamber 4 is forced through the annular gap 23 and through the openings into the cylinder chamber 25, which means that also the piston 18 and the piston rod 19 thereof are displaced to the right, bringing along the outer telescopic part 38. Return oil ahead of the piston 18 is drained through the outlet port 21 to the pressure vessel. 
     Because of the resistance which the oil encounters in the annular channel 23 is ensured that the cylinder 1 will have completed its outwards movement before the piston 18 has completed its outward movement. This order of movements is important in order to allow a load limit device incorporated into the crane to operate in the intended manner without risk of excess load on the comparatively weak piston rod 19. 
     When the telescopic parts 34 and 38 are to be returned to their initial positions (FIG. 4), pressurized oil is introduced through the inlet port 21. The piston 18 together with the piston rod 19 thereof are thereby forced to the left, bringing along the outer telescopic part 38. Return oil is forced from the space 25 via the openings 24 and the annular gap 23 into the annular chamber 4 in the space 39 between the annular piston 13 and the end section 10, and is drained through the outlet port 12 to the pressure vessel. At the same time--or preferably a brief period after the application of the oil pressure in the inlet port 21--oil is forced through the inlet port 11 into the annular chamber 4, with the result that the cylinder 1 is displaced to the left from the position illustrated in FIG. 2, bringing along its telescopic part 34 (FIG. 4). 
     In order to ensure that as a consequence of this return movement also piston 18 and the piston rod 19 thereof complete their inward movements before the cylinder 1 has completed its inward movement, it is preferable to provide valves in the inlet and outlet ports 11, 12, 21 to regulate the pressure and the flow, which is easily done with the structure in accordance with the invention. 
     In order to allow evacuation of the air present in a space 40 formed in the piston rod 14 between the end piece 26 thereof and the bottom section 17 of the inner jacket 3, when the cylinder 1 is displaced to the left, the end piece is provided with a channel 41 which is normally closed by a spring-loaded ball valve 42 but opens at the occurrence of an excess pressure in the space 40. 
     The hydraulic cylinder 1 as illustrated and described is to be regarded as an example only and its various parts may be structurally altered in a variety of ways within the scope of the invention. The piston 18 may be replaced by an annular piston arranged in an annular chamber in a manner similar to the one described above concerning the annular piston 13. It is likewise possible to arrange an annular piston of this kind between the annular piston 13 and the piston 18 in order to be able to increase the number of the telescopic parts 34 and 38. The hydraulic cylinder in accordance with the invention may be used for other purposes than telescopic crane booms.