Abstract:
A road marking machine comprising a spray gun and a combination of two displacement pumps for supplying the marking substance to the spray gun, wherein the combination is driven in proportion with the traveling speed and wherein the pumps, each after starting the positive displacement operation thereof, which starts during the supply operation of the corresponding other displacement pump, pre-compress the marking substance, then stops and continues the pressure stroke when the other displacement pump ends its supply, with hydraulic cylinders for driving the displacement pumps.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a road marking machine having at least one spray gun for a pumpable marking substance, such as paint, and a combination of two displacement pumps for dosably supplying the marking substance to the spray gun, wherein the combination of the two displacement pumps is driven in proportion with the traveling speed of the road marking machine. 
     Such a road marking machine is known from German patent application DE 30 07 116 C2. A serious disadvantage of that machine is the branched guiding of the paint streams after leaving the supply chambers of the pump, with the stream guided by valves. The 3/2-port-valves must be positively controlled in that their functional principle is for dividing the interrupted paint streams discharged from the supply chambers into a pulse free paint stream for the using units, and an interrupted remaining paint stream to be recycled to the reservoir. These valves are subject to high wear, in particular with abrasive paints. Furthermore, the valves are complicated and expensive with increasing operating pressures. 
     Besides the disadvantage associated with recycling a part of the paint stream to the reservoir, it is an additional disadvantage that this part paint stream has to be guided through a throttle, and the resistance of the throttle has to be adjusted to the resistance of the spray gun. Positively controlled valves for the control of paint streams are subject to high wear, in particular when the paints contain solid particles, and are complicated and expensive for high pressures. The same is true for the throttle. 
     It is a further disadvantage that the pump has to be driven even when the marking lines are interrupted, and that during the interruption the paint stream dedicated for the spray gun has to be recycled back to the reservoir through a valve to be opened and a throttle adjusted to the resistance of the spray gun. 
     This constant pumping is necessary to have the required spray pressure ready when the spray gun is opened again when a new part of the marking line is started. 
     The branched guidance of the paint with many paint contacting members is also a disadvantage when the unit has to be designed in stainless steel for paints being aggressive for normal steel, which again will greatly increase the manufacturing costs. 
     The process known from the patent document cited above has the further disadvantage that the two displacement members are mechanically driven through cam plates. The economic transfer of forces in this manner is limited for the sizes of the utilized pumps. Pumps of this type have been known only for the low pressure range with spray pressures up to 15 bar. For the high pressure range with pressures up to 200 bar, such a mechanical drive would be very heavy, very complicated, and costly for the higher forces involved. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to further develop a generic road marking machine, such that with a simplified construction of the pump combination, a trouble free and almost maintenance free operation is provided. This object is attained with a generic road marking machine in which a pump combination is provided comprising two single oscillating displacement pumps, wherein the single pumps, each after starting the positive displacement operation thereof, which starts during the supply operation of the corresponding other displacement pump, pre-compresses the paint with an outlet valve closed up to a pressure close or equal to the supply pressure of the supplying displacement pump, then stops while holding the pre-compression final pressure reached, and continues the displacement operation while supplying the paint only when the corresponding other displacement pump ends its supply. The term positive displacement in this context is a movement of the displacing element decreasing the volume of the pump supply chamber. The pulsing, when transferring the supply from the one single pump through the other pump, therein depends on the difference between supply pressure and the end pressure reached at the pre-compression, and is very small. The term marking substance includes various substances, but as a specific example useful in an embodiment of the invention described herein, the term paint will be used throughout this disclosure, not in a limiting sense, but rather merely as a specific example. 
     The control for attaining a low pulse supply is attained at the drive side of the displacing members with pumps of such a supply operation. The total paint volume leaving the pump supply chambers is pumped to the spray pistol or may be reserved for the spray pistol, respectively. The paint flow path system is simple. Other than the check valves associated with the pump supply chambers, no further control valves, which are complicated and subject to wear for deviating the paint streams, are required, and no throttles for influencing the pressure are required either. The number and complexity of the paint contacting construction members is small, leading to lower cost when adapting the materials to the requirements of the paint. 
     The pumps of the road marking machine are driven such that the supply volume, when changing the traveling speed, will vary in the same ratio as the traveling speed. With varying paint volumes, the supply pressure will also vary, as the outlet cross-section of the spray pistol remains the same. With varying supply pressure, the difference between supply pressure and fixedly adjusted pre-compression final pressure will also vary, and thereby the amplitude of the supply volume pulse when transferring the supply from the one pump supply chamber to the other will vary. Pulses, however, generate thin areas or interruptions in the marking line to be generated and have to be avoided for this reason. 
     According to the invention, an automatic adaption of the pre-compression final pressure to the supply pressure is attained. 
     By means of the supply pressure of the currently supplying single pump, up to the end of the displacing operation thereof, the end pressure of the pre-compression of the other single pump is controlled to a value in proportion with the supply pressure. 
     According to the invention, the supply pressure of the single pump currently supplying to the spray gun is used as a control value for the device which interrupts the displacement operation of the other single pump as it reaches a pre-compression pressure, defined by the supply pressure, while holding this pressure. 
     During the interruption of a line, i.e., with a closed spray gun, the paint pressure has to be held at a pressure which corresponds to the spraying pressure of an opened spray gun during the application of a line, in order to have the spray pressure ready, which is required for a good spray quality at the beginning of the spraying at the start of a further line part. This is attained, according to an embodiment of the invention, such that when the spray gun is closed, the oil stream for driving the pump finds a side exit open through which the oil stream is guided into the oil reservoir through a throttle generating the required pressure, wherein the pump will be stopped. The counter-pressure generated by the throttle therein is depending on the size of the oil stream being in proportion to the traveling speed, in the same manner as the paint pressure generated during the pump operation through the nozzle of the spraying gun is dependent on the size of the paint stream being in proportion with the traveling speed. 
     The invention is now further explained referring to a drawing depicting the example of a combination of two displacement pumps wherein the positive and negative displacement is attained by rigid displacing members displaceable in cylinders. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates the total design of the road marking machine, in principle. 
     FIG. 2 schematically illustrates the construction of the pump combination from FIG. 1, in principle. 
     FIG. 3 illustrates a side sectional view of one of the two pressure controllers of FIG. 2 for one of the two hydraulic streams to the hydraulic cylinders. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, displacement pumps 1a and 1b combined in a pump combination 100, are driven by two oil streams through ducts 108a and 108b, with the oil streams, in the size thereof, proportional with the traveling speed and equal with each other. The generation of these two oil streams from one oil stream, supplied by a hydraulic pump 104 driven by an internal combustion engine 103, is attained in a control unit 105, which is not further explained, while supplying a traveling speed signal which, e.g., is derived from a wheel 106 rolling on a road. 
     The backstream of the hydraulic oil from the pump combination 100 to an oil tank 107 is attained through a duct 15. A side outlet duct 109a or 109b, respectively, is connected to the ducts 108a and 108b to the pump combination 100, with the side outlet lines leading to 2/2-port-valves 110a or 110b, respectively, wherefrom again ducts lead to throttles preferably to adjustable throttles 111a or 111b, respectively. The outlet sides of these throttle valves are connected with the duct 15. 
     The pump combination 100 sucks paint from a reservoir 112 and supplies it through a duct 8 to a spray gun 113. Simultaneously with the closing of the spray gun 113 the valves 110a and 110b are opened. After closing the spray gun, when the pump combination 100 stands still, the oil streams being supplied through the ducts 108a and 108b now are discharged through the ducts 109a or 109b, respectively, through the opened valves 110a or 110b, respectively, and subsequently through the throttles 111a or 111b, respectively, to the oil tank 107. The throttles must be adjusted in such a way that the oil pressure acting in the ducts 109a or 109b, respectively, and therefore in the non-actuated pump combination 100, corresponds to the oil pressure which occurs when the spray gun 113 is open and the pump combination 100 is working. 
     Referring to FIG. 2, the pump combination 100 of FIG. 1, comprising the two displacement pumps 1a and 1b, is explained: 
     The displacement pumps 1a, 1b comprise displacing members 2a, 2b which are connected with pistons 3a, 3b of hydraulic cylinders 4a, 4b, such that a movement of the hydraulic pistons 3a, 3b is transferred to the displacing members 2a, 2b. With a movement of the pistons 3a, 3b, because of oil supply to cylinder chambers 5a, 5b, the paint which is contained in the pump chambers 6a, 6b is compressed, and after reaching the pressure acting in the duct 8 to the spray gun 113, is pressed through an outlet valve 7a, 7b into the duct 8. The hydraulic oil is fed to the cylinder chambers 5a, 5b through ducts 9a, 9b. For accomplishing the suction stroke, the hydraulic oil is supplied to cylinder chambers 10a, 10b. The control of the oil streams to the cylinder chambers is accomplished by means of valves 11a, 11b which are fed with the oil through ducts 108a, 108b. Each of the two displacement-piston-combinations 2a, 3a and 2b, 3b oscillates between an upper and a lower reverse position. Switches or sensors 18a, 18b and 19a, 19b in the reverse position sense the position of the corresponding displacement-piston-combination and control the oil streams by switching the corresponding hydraulic valves 11a, 11b such that the stroke is reversed. The upwards stroke is the pressure or supply stroke, respectively. The downward stroke is the suction stroke. 
     Ducts 12a, 12b are connected with the ducts 9a, 9b. The ducts 12a, 12b lead to pressure controllers 14a, 14b and then to the common duct 15 through connection ducts 15a, 15b back to the oil tank 107. The pressure controllers 14a, 14b are designed such that an oil stream is passed only when a certain pressure is reached, with the pressure defined by the pressure in the cylinder chambers 5a or 5b, respectively, of the pump 1a or 1b, respectively, supplying just into the duct 8. For this reason the pressure controllers 14a, 14b are connected with control pressure ducts 17a, 17b to the cylinder chambers 5a, 5b. 
     The pressure controller 14a associated with the pump 1a is connected with the cylinder chamber 5b of the pump 1b through the duct 17a, and the pressure controller 14b of the pump 1b is connected with the cylinder chamber 5a of the pump 1a through the duct 17b. 
     Shortly prior to the switch 19a, 19b enacting the reverse of the supply stroke to the suction stroke, there is a further switch 20a, 20b. The object of this switch is to enact an interruption of the oil stream through the ducts 12a or 12b, respectively, to the oil tank, and that of the other pump standing still at the end of the pre-compression stroke. Therefore, the switch 20a is dedicated for the interruption of the oil stream through the duct 12b, and the switch 20b is dedicated for the interruption of the oil stream through the duct 12a. 
     The pump 1b is illustrated in the supply operation, i.e., it supplies paint through the opened outlet valve 7b into the duct 8 to the spray gun 113. The pressure acting in the cylinder chamber 5b and therefore in the ducts 9b, 12b and 17a depends on the pressure of the paint in the pump chamber 6b. 
     The pump 1a is illustrated standing still after the pre-compression stroke is finished. The pressure controller 14a is in the opened position such that the hydraulic oil streaming into the duct 9a is discharged through the duct 12a to the duct 15. Therein, the pressure in the duct 9a, and therefore in the cylinder chamber 5a, and also the pressure in the pump chamber 6a, is maintained at a value by the pressure controller 14a which is defined by the geometric design in the pressure controller 14a and by the pressure of the cylinder chamber 5b of the pump 1b acting as a control pressure through the duct 17a onto the pressure controller. 
     When the displacing member 2b, during the supply stroke, reaches the switch or sensor 20b, this switch or sensor enacts the interruption of the oil stream through the duct 12a. This may be accomplished by different means, e.g., by check valves in the ducts 12a, 12b. In the example illustrated, this is accomplished by enacting an additional force onto the valve slider of the pressure controller 14a, whereby the pressure controller is closed. Thereupon, the displacing member 2a continues its pressure stroke now as a supply stroke, wherein paint is pressed or forced through the opening outlet valve 7a into the duct 8 to the spray gun 113. 
     When the displacing member 2b, at the end of the supply stroke thereof, reaches the lower dead point, the suction stroke is triggered by the switch 19b, whereupon the outlet valve 7b closes and the paint streams through an opening inlet valve 21b (or 21 a in the left pump 1a) into the chamber 6b. 
     By means of adjusting the distance between the switches 20b and 19b, a lag between the switch signal and completed valve switch may be compensated such that a continuous supply stroke of the pump 1a, to the ending supply stroke of the pump 1b, is accomplished. 
     When the displacing member 2a is in the supply stroke, the displacing member 2b enacts the suction stroke because the supply of hydraulic oil into the cylinder chamber 12b, with the velocity of the suction stroke, is larger than that of the pressure stroke. The switch or sensor 18b, respectively, enacts the completion of the suction stroke, and a transfer to the pressure stroke, which begins with the pre-compression of the paint. The oil pressure resulting in the duct 9b to the cylinder chamber 5b is also present in the pressure controller 14b through the duct 12b. The pressure controller remains closed up to reaching the opening pressure because the oil pressure from the cylinder chamber 5a of the pump 1a enacting the supply stroke with the oil pressure being the control pressure acting on the pressure controller through the duct 17b. The size of the opening pressure, as has been explained before, is defined by the geometric design in the pressure controller and by the control pressure. After opening the pressure controller 14b, the hydraulic oil supplied through the duct 9b streams through the duct 15 to the oil tank 107, while holding the opening pressure. 
     In FIG. 3, one of the two pressure controllers which have the same function is illustrated, in an example of the pressure controller 14a. In a housing 25a there is a slider 26a containing, at the end thereof, a conical closing member 27a. This conical member closes an opening 28a, with a cross section A 2 , which is connected with the duct 12a and thereby with the hydraulic cylinder chamber 5a. In direction towards the closing member 27a, the opening 28a is enlarged to form a chamber 29a. The chamber 29a is connected with the duct 15 leading to the oil tank 107. The end of the slider 26a facing away from the closing member 27a has an effective cross-section A, and together with the housing 25a forms a chamber 30a. This chamber 30a is connected with the duct 17a and thereby with the hydraulic cylinder chamber 5b of the pump 1b, with the pump supplying paint into the duct 8 to the spray gun 113. 
     Furthermore, the housing 25a comprises a chamber 31a with a piston 32a. The latter is connected with a piston rod 33a which projects in a sealing fashion towards the housing 25a into the chamber 30a and presses onto the front face of the slider 26a when the chamber 31a is under pressure. The hydraulic oil is supplied to the chamber 31a through a duct 34a. 
     By the oil pressure in the chamber 30a, a force F 1  equaling A 1  ×p.sub.(5b) acts on the slider 26a, wherein P.sub.(5b) is the hydraulic pressure of the cylinder chamber 5b. In the opening 28a, a force F 2 , equaling A 2  ×p.sub.(5a), in direction opposite to F 1  acts on the slider, wherein p.sub.(5a) is the hydraulic pressure of the cylinder chamber 5a of the pump 1a accomplishing the pre-compression stroke. While the force F/ 2  increases from zero with increasing pre-compression in the pump chamber 6a, and therefore with correspondingly increasing hydraulic pressure p.sub.(5a), the force F 1  has a constant value. When the force F 1  is equal to F 2 , the closing force for the opening 28ais zero and the hydraulic oil begins to stream or flow from the opening 28a through the chamber 29a to the duct 15a when the closing member 27a opens. The accomplished oil pressure corresponds to the end pressure of the pre-compression of the paint in the pump chamber 6a. Now the piston displacement combination 3a, 2a comes to a standstill. The furthermore streaming oil through the duct 9a will then stream through the pressure controller 14a to the duct 15, wherein the oil pressure in the opening 28a is held constant by the force F 1  acting on the opening. The ratio of the pre-compression end pressure of the pump 1a to the supply pressure of the pump 1b is defined by the area ratio A 1  :A 2  and is not dependent on the value of the supply pressure. 
     When the oil stream through the opening 28a has to be stopped, the pressure control chamber 31a is impinged with pressure by the switch 20b through the duct 34a, such that an additional force results which is required for closing the opening 28a. The piston displacement combination 3a, 2a then enacts the pressure stroke as a supply stroke beginning from the stand-still situation. By a control, which is not further illustrated, the impingement of the pressure control chamber 31a with pressure is held at least up to the end of the supply stroke. 
     When the outlet valves 7a, 7b are designed as automatically opening valves, e.g., as illustrated as check valves by means of the area ratio A 1  :A 2 , the pre-compression end pressure has to be selected smaller than the supply pressure. Otherwise the valves 7a, 7b would be opened against the supply pressure acting in the duct 8 as a consequence of the pre-compression pressure increasing above the supply pressure. This would mean that in this case both pumps would supply into the duct 8. In the case that the outlet valves 7a, 7b are provided as positively controlled valves with the control enacted by additional energy, or, when with an additional energy, an additional closing force has to be provided, then the pre-compression end pressure has to be selected equal to or larger than the supply pressure. 
     As is apparent from the foregoing specification, the invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. It should be understood that I wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of my contribution to the art.