Study on energy-saving composite cylinder hydraulic lifting system*

Lu-jun ZHANG，Zhi-jiang LIN

1School of Electromechanical Automobile Engineering，Yantai University，Yantai264005，China；2Key Laboratory of Advanced Manufacturing and Control Technology in Universities of Shandong，Yantai 264005，China；3Wendeng Agricultural Machinery Service Station，Wendeng 264400，China

Study on energy-saving composite cylinder hydraulic lifting system*

Lu-jun ZHANG?1，2，Zhi-jiang LIN3

1School of Electromechanical Automobile Engineering，Yantai University，Yantai264005，China；2Key Laboratory of Advanced Manufacturing and Control Technology in Universities of Shandong，Yantai 264005，China；3Wendeng Agricultural Machinery Service Station，Wendeng 264400，China

The construction machines，such as the crane，hydraulic grab and etc．，utilize the hydraulic lifting system．The conventional hydraulic lifting system usually can not recover the potential energy of the heavy object．In order to avoid the object falling too fast，a throttle valve is set in the oil circuit to limit the falling velocity o f the object．Therefore，the potential energy will be converted into the thermal energy，which not only brings the energy loss and also could cause some thermal problem s．In order to avoid the loss of energy，an energy-saving composite cylinder hydraulic lifting system is designed．This paper presents the basic structure and working theory of this new hydraulic lifting system．By utilizing composite cylinder with special structure combined with accumulator，this system can limit the lowering velocity of heavy object and also could recover the potential energy of the object being lowered．Thus，this system has remarkable effect of energy-saving．The parameters of the accumulator used in this system are determined．The mathematical model of lifting condition is proposed，and the simulation analysis of lifting condition is conducted as well．

Composite cylinder，Accumulator，Energy-saving，Hydraulic lifting system，Potential energy recovery

1．Introduction

Lots of engineering construction machines，such as the crane，hydraulic grab，hydraulic elevator and hydraulic pumping units，have the hydraulic lifting system［1-4］.The hydraulic lifting system lifts and lowers the heavy object continually.When lifting the heavy object to a high position，the system will store large amount of potential energy.However，when lowering the heavy object down，the potential energy will be released.The conventional hydraulic lifting system can not recover this kind of potential energy. In order to avoid the object falling too fast，a throttle valve is usually adopted in the oil circuit to limit the falling velocity.Therefore，the potential energy will be converted into the thermal energy，which will not only lead to the energy loss and also causes the thermal problems［5-6］.In order to avoid the loss of energy，an energy-saving composite cylinder hydraulic lifting system is designed.By utilizing the composite cylinder with special structure combined with accumulator，this new hydraulic lifting system not only could limit the lowering velocity of heavy object，but also could recover the potential energy of the lowered object.Therefore，this system has remarkable characteristics of energy-saving.

2．Basic structure of the energy-saving composite cylinder hydraulic lifting system

The two types of composite cylinder for hydraulic lifting systems are shown in Figure1 and Figure 2，respectively.As shown in Figure1，the composite cylinderⅠis composed of one piston cylinder and one plunger cylinder.The large piston rod of the piston cylinder is adopted to lift the heavy object，and the small piston rod of the piston cylinder is also used as the plunger of the plunger cylinder.Thus，the composite cylinderⅠis divided into three chambers（i. e.，Q1，Q2，and Q3），whose acting areas are not equal.As shown in Figure 2，the composite cylinderⅡis also composed of one piston cylinder and one plunger cylinder，but the combining type of the piston cylinder and plunger cylinder is not the same. The plunger of the plunger cylinder is also used as the cylinder of the piston cylinder，however，the piston rod of the piston cylinder is fixed on the base of the plunger cylinder.Thus，the composite cylinderⅠis also divided into three chambers（i.e.，Q1，Q2，and Q3），whose acting areas are not equal either.For the above hydraulic lifting systems based on two types of composite cylinder，the oil inlet of Q1 chamber is connected to the accumulator，and the oil inlets of Q2 chamber and Q3 chamber are connected to two oil inlets of the directional valve 6，respectively.Through the direction-changing of the directional valve 6，Q2 chamber and Q3 chamber are connected to high pressure oil and low pressure oil alternately. The relief valve 2 plays the function as safeguard，and when the oil pressure in the system surpasses the setting value，this valve will open and overflow the oil.The reducing valve 4 controls the minimum pressure of the circuit of Q1 chamber-accumulator，and the circuit of the Q1 chamber-accumulator is supplemented the oil through this valve.The relief valve 3 controls the maximum pressure of the circuit of the Q1 chamber-accumulator，and thus makes the pressure of this circuit in a certain range.

3．Working principle of the energy-saving composite cylinder hydraulic lifting system

Because the working principle of two types of composite cylinder hydraulic lifting system is similar， the following will take the hydraulic lifting system based on the composite cylinderⅠas an example to conduct the analysis.

When lifting the heavy object at the first time，the directional valve 6 is set at the right position.The oil pump supplies the oil to Q2 chamber，and the oil in Q3 chamber will flow back to the oil tank.At this moment，since the accumulator is not charged into the oil，the circuit of the Q1 chamber-accumulator is in low pressure state，and the reducing valve 4 opens.Thus，the oil pump also supplies the oil to Q1 chamber as well，and the large piston rod 8 will go up to lift the heavy object.

During the process of lowering the heavy object，the directional valve 6 is set at the left position.The oil pump supplies oil to Q3 chamber，and the oil in Q2 chamber will return to the oil tank.Due to the weight of heavy object and the oil pressure，the large piston rod 8 will go down.The oil in the Q1 chamber will be discharged into the accumulator，and the air in the accumulator is compressed and the energy is stored.At the same time，because of the action of the accumulator，the oil in the Q1 chamber keeps a certain pressure and gives the system a balancing resistance，thus prevents the heavy object from falling too fast.

When lifting the heavy object again at next time，the directional valve 6 will be set at the right position.The oil pump supplies oil to Q2 chamber，and the oil in Q3 chamber will return to the oil tank. At the same time，the accumulator releases the recovered energy from the last time to Q1 chamber，and this amount of energy together with the pressure oil in Q2 chamber will drive the large piston rod to go up to lift the heavy object.When the directional valve 6 is set at the middle position，the heavy object can stop at any location.By adjusting the displacement of the variable pump 1，the speed of lifting and lowering the heavy object could be adjusted.

4．Determination of the parameters of the accumulator

The following condition must be satisfied during the lifting process of heavy object

Where，ppis the pump pressure，p1is the lowest pressure of the accumulator，A1and A2are the acting area of the oil pressure in Q1 and Q2 chamber，respectively，W is the weight of the heavy object.

If the pressure of ppis constant，the values of p1and A1could be determined by Eq.（1）.After A1has been determined，the oil volumeΔV that the accumulator needs to supply could be evaluated as follows：

Where，s is the stroke of large piston rod in Figure 1 or plunge in Figure 2.

Figure 1.Hydraulic lifting system based on composite cylinderⅠ

Figure 2.Hydraulic lifting system based on composite cylinderⅡ

Then the total volume V0of the accumulator is［7］

Where，p0isthe charging pressure of the accumulator，p2is the highest pressure of the accumulator.

Once the p1is determined，p0and p2could be obtained through the formula 0.9 p1＞p0＞0．25 p2．

5．Mathematical model of lifting condition

In order to analyze the mathematical model of lifting condition，the pressure loss of oil through pipe and each directional valve will not be considered. According to Newton’s second law，the dynamic equation of lifting part could be obtained as follows：

Where，ppis the pump pressure；p is the pressure of the accumulator；A1and A2are the acting area of the oil pressure in Q1 and Q2 chamber，respectively；W is the weight of the heavy object；m is the weight of the heavy object；v is the velocity of the heavy object.

Because the accumulator could provide a large amount of pressure oil for the system in a short time，the volume of air in the accumulator will vary according to the adiabatic process，and then［7］

Where，piis the instantaneous pressure of the accumulator air，Viis the instantaneous volume of the accumulator air，p0is the accumulator air charging pressure，V0is the volume of the air bag.

If the inertia of air bag is not considered，then

When the lifting displacement of the heavy object is x，the instantaneous volume of the accumulator air could be evaluated as follows：

Combining（5），（6）and（7），one could obtain：

Plug Eq.（8）into Eq.（4），it will yield：

Eq.（9）is the mathematical model of lifting condition.

6．Simulation analysis of lifting condition

Eq.（9）is an nonlinear differential equation，and it could not be solved by analytical methods. Therefore，a program is designed to conduct the numerical simulation analysis.The known data are as follows：s=8 m，p0=10 MPa，pp=10 MPa，A1= 0.005 m2，A2=0.005 m2，V0=0.069 m3.

Substitute these data into Eq.（9）and conduct the numerical integration by using Matlab，one could obtain the relationships between the heavy object lifting displacement x，lifting velocity v and the time t with different weight of object W，which are shown inFigure 3 and Figure 4，respectively.From these two figures，it could be seen that during the lifting process of heavy object，the heavier the object，the lower the average lifting velocity is，and the longer the lifting time is.

Figure 3.Curves of the lifting displacement under W=70 kN，85 kN and 95 kN

Figure 4.Curves of the lifting velocity under W=70 kN，85 kN and 95 kN

7．Conclusion

By adopting the composite cylinder with special structure and accumulator，the energy-saving hydraulic lifting system can not only limit the lowering velocity of the heavy object and also could recover and reuse the potential energy of the heavy object.Therefore，this kind of lifting system could save energy remarkably.In addition，the system will work more smoothly by using the accumulator.This kind of energy-saving hydraulic lifting system could be widely used in lots of machinery with hydraulic lifting system.The mathematical model of lifting condition is deduced，and simulation analysis of lifting condition is conducted as well.

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節(jié)能組合油缸液壓升降系統(tǒng)研究*

張路軍?1，2，林治江3

1.煙臺大學機電汽車工程學院，山東煙臺 264005
2.山東省高校先進制造與控制技術(shù)重點實驗室，山東煙臺 264005
3.文登市農(nóng)機技術(shù)推廣服務(wù)站，山東文登 264400

建筑機械如汽車起重機、液壓挖掘機等都采用了液壓升降系統(tǒng)。常規(guī)液壓升降系統(tǒng)不能回收重物下放的重力勢能，為避免超速下降，一般在下降的回油路中設(shè)置節(jié)流閥等限速元件。重力勢能被轉(zhuǎn)化成熱能，不僅造成很大的能量損失且使油液的發(fā)熱量增多。為避免這種能量浪費現(xiàn)象，設(shè)計了一種節(jié)能組合油缸液壓升降系統(tǒng)。介紹了這種液壓升降系統(tǒng)的基本結(jié)構(gòu)和工作原理。該系統(tǒng)將特殊結(jié)構(gòu)的組合油缸和蓄能器相結(jié)合，可以限制下降重物的速度，同時能回收利用重力勢能，節(jié)能效果顯著。確定了蓄能器的參數(shù)，推導出了起升工況數(shù)學模型，進行了起升工況仿真分析。

組合油缸；蓄能器；節(jié)能；液壓升降系統(tǒng)；勢能回收

TG502.32

10.3969／j.issn.1001-3881.2014.18.012

2014-05-01

*Project supported by Technological Development Planning Fund of Shandong Province（2013YD03058）

?Lu-jun ZHANG，PhD.E-mail：zlj6910＠163.com