Pumps – Alternate series or parallel operation of plural pumps
Reexamination Certificate
1999-09-21
2001-03-06
Freay, Charles G. (Department: 3746)
Pumps
Alternate series or parallel operation of plural pumps
C417S295000, C417S571000
Reexamination Certificate
active
06196806
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a continuously variable transmission or CVT according to the preamble of claim
1
.
DESCRIPTION OF THE RELATED ART
Such a CVT is disclosed in the patent publication EP-A-0 826 910 and is used in particular for motor vehicles. The transmission ratio of the CVT is hydraulically continuously adjustable in a certain range of transmission ratios. The usual requirement for the known CVT is that the volume flow and the pressure of the hydraulic medium needed to control the CVT must be adjustable within a wide range. A high volume flow is needed to effect a substantial change in the transmission ratio within a short time, whilst a low volume flow suffices for maintaining the transmission ratio and the lubrication of the CVT. The power taken up by the pumps is partly determined by the magnitude of the volume flow delivered by the pumps, as a result of which the preference is, in principle, to allow the pumps to deliver as low as possible a volume flow. In order to be able to comply with the said requirement efficiently, in the known construction the hydraulic circuit is provided with hydraulic adjustment means for switching the pumps in series or in parallel. When the pumps are switched in parallel the volume flows delivered by the individual pumps are combined to give one high volume flow, whilst when the pumps are switched in series, the volume flow delivered by a first pump is fed to a second pump. Thus, when the pumps are switched in series a low volume flow is delivered, which is equal to at most the lowest volume flow delivered by a pump. The power taken up by the pumps connected in series is thus significantly lower than the power taken up by the pumps switched in parallel. By operating the pumps switched in series when possible, the CVT is utilized efficiently.
In the known CVT the inlet openings of the first and the second pump are connected to a reservoir for hydraulic medium and the outlet openings of the first and the second pump are connected to an outflow channel. In this construction the hydraulic adjustment means consist of a switch valve, by means of which the outlet opening of the first pump can be connected to either the inlet opening of the second pump or the outflow channel. In the former case the pumps are switched in series, the switch valve also connecting the inlet opening of the second pump to the reservoir. The known switch valve is provided with four hydraulic ports. A first port is connected to the outflow channel, a second port is connected to the outlet opening of the first pump, a third port is connected to the inlet opening of the second pump and a fourth port is connected to the reservoir. The switching element of the switch valve is provided with two annular recesses such that when the pumps are switched in series the second and the third port are hydraulically connected to one another via an annular recess, whilst the first and fourth port are closed off by the switching element, and when the pumps are switched in parallel both the first and the second port and also the third and the fourth port are hydraulically connected to one another via an annular recess. The known switch valve is furthermore also provided with a fifth port connected to the reservoir for discharging surplus hydraulic medium.
The known CVT operates satisfactorily but has the disadvantage that brief but violent pressure variations in the hydraulic circuit occur during use of the pumps. Such pressure variations disturb the operation of the CVT and also produce an annoying noise. Moreover, a high force is required to change the position of the switching element since high volume flows under a high pressure have to be closed off and/or diverted during switching of the switch valve. Moreover, as a construction, the switch valve used in the known CVT has the disadvantage that it is elongated, because it is provided with five ports separated from one another. The known switch valve is further provided with a relief valve, for which a bore that is difficult to produce is required.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a CVT in which undesirable and interfering pressure variations have been largely overcome and in which the adjustment means can be produced advantageously. According to the invention a CVT of this type is obtained with the aid of the adjustment means according to the below disclosure.
A CVT according to the invention has the advantage that the hydraulic connections needed for switching the pumps in series and in parallel are made and broken by at least two separate valves. By means of the measure according to the invention it becomes possible for the valves to be opened more or less simultaneously during switching of the pumps. The opening and closing of the ports consequently proceeds sequentially and automatically, without abrupt changes in pressure. Furthermore, valves of simple construction can be used.
In a particular embodiment of the invention the switching means comprise a non-return valve and a switch valve. The switch valve is provided with three hydraulic ports, there always being two of these connected to one another in the two extreme positions of the switch valve. With this arrangement the switching element of the switch valve is so designed that all three ports can temporarily be in communication with one another during switching of the switch valve. Preferably, the switch valve is used for switching the outlet opening of the first pump between the inlet opening of the second pump and the outflow channel. The non-return valve is then mounted between the inlet opening of the second pump and the reservoir, such that hydraulic medium is able to flow from the reservoir to the inlet opening of the second pump. In the adjustment means according to the invention no pressure build-up takes place during switching of the switch valve because the said valves are opened more or less simultaneously. Consequently switching of the pumps proceeds smoothly and without abrupt changes in pressure.
According to a further development of the invention, the switch valve is electromagnetically controllable. As a result it is possible to allow switching of the pumps to take place with the aid of electronic means, for example as a function of transmission parameters such as the speed of revolution of the input shaft of the CVT or the rate at which the transmission ratio changes. It is then always possible to switch the pumps in parallel, so that a high volume flow can be delivered at the desired point in time. If the switch valve is hydraulically damped, for example with the aid of a restriction, the transition between the two extreme positions of the switch valve proceeds extremely uniformly, whilst the electromagnetic control can still consist of a simple on/off control.
According to yet a further development of the invention, a non-return valve is pretensioned by a spring. What is achieved by pretensioning a non-return valve is that said valve allows the passage of hydraulic medium only when there is a certain pressure drop in the direction of passage over the non-return valve. A non-return valve can advantageously be provided with hydraulic damping means which allow the opening and closing of the non-return valve to proceed uniformly.
A significant ancillary advantage of switching the pumps in series is that the occurrence of cavitation close to the inlet opening of a second pump can be largely if not completely prevented. Cavitation occurs if the pressure of the hydraulic medium falls below a critical value and is associated with undesirable effects, such as the production of noise and wear of pump components. In order to prevent cavitation in the inlet opening of the second pump, the volume flow delivered by the first pump is adjusted such that it is always greater than the volume flow demanded by the second pump. As a result, when the pumps are switched in series an overpressure is built up in the inlet opening of the second pump, as a consequence of which the pressure in the inlet opening i
Freay Charles G.
Gray Michael K.
Van Doorne's Transmissie B.V.
Young & Thompson
LandOfFree
Continuously variable transmission does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Continuously variable transmission, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Continuously variable transmission will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2512577