Hidrolik kontrol valflerinde çalışma kuvvetleri

Abstract

Bu araştırma, kontrol valf pistonlarına çeşitli çalışma şartları altında galen kuvvetlerin incelenmesi amacıyla yapıl mıştır. Çalışmada, nominal çapı 15 mm.- olan kritik merkezli, ka palı merkezli ve açık merkezli üç değişik tipte hidrolik kontrol valfi esas alınarak, bu valfler dar toleranslarda- imal- edilmiş tir. Bu valf ler in pistonlarında meydana gelecek olan çalışma kuvvetlerini incelemek için çok amaçlı Özel bir deney tesisatı geliştirilmiş ve deneyler gerek statik, gerekse dinamik çalışma şartlarında gerçekleştirilmiştir. Kontrol valflerinde meydana gelen akış kuvvetlerinin doğ ru olarak incelenmesi için, önce vaiflerin aynı şartlardaki akış karakteristikleri deneysel olarak tespit edilmiştir. Yapılan de neyler sonunda, vaiflerin ±500 ym lik bir piston yerdeğişimi bölgesinde lineer bir debi karakteristiğine sahip olduğu görül düğünden, çalışmaların büyük bir kısmı bu. bölge içinde yapılmış tır. Ayrıca valf boşaltma katsayılarının çalışılan bölgede sabit olmadığı anlaşıldığından, 40 ve 60 bar besleme basınçlarında boşaltma katsayılarının piston yerdeğişimi ve Reynolds sayısına göre değişimleri elde edilmiştir. Gerek basır.çsiîi, gerekse 20, 40 ve 60 bar beslema basınç larında valflerde meydana gelen dinamik ye statik sürtülme kuv vetleri incelenmiştir. Yapılan çalışmalar esnasında valf piston- lc.rıi-da radyal kuvvetlerin meydana geldiği, bunun da vaLflerdeki sürtünme kuvvetlerinin artmasına sebep olduğu görülmüşt ir. Valf pistonlarının hareketsiz olduğu anlarda sürtünme kuvvetlerinin daha da büyük' mertebelere ulaştığı anlaşıldığından; valf piston- larmdaki statik kopma kuvvetleri geniş olarak incelenmiştir. Yapılan deneyler sonunda valflerdeki statik kopma kuvvetlerinin, kopmanın gerçekleşmesinden sonra ulaşılan, Coulomb sürtünme kuv vetlerine oranı kapalı merkezli valfde takriben (3,5), kritik ve açık merkezli valflerde `ise (4) olarak tespit edilmiştir.- IV - Valf lerde`` meydana gelen daimi akış kuvvetleri ölçülmüş ve bu kuvvetlerin, piston yerdeğişimine de bağlı olarak, teorik de ğerlerden daima.o ir miktar büyük olduğu görülmüştür. Bunun, ger çek valflerdeki sukbe köşelerinin ideal keskinlikte olmamasından ötürü, meydana gelen jet akış açısının teoriden bir miktar daha küçük olmasında^ kaynaklandığı anlaşılmıştır. Valf pistonlarının sinusoidal dinamik tahriki esnasında meydana gelen çalışma kuvvetleri kapsamlı olarak incelenmiştir. Yapılan çalışmalar sonunda, çeşitli frekanslarda ölçülmüş olan çalışma kuvvetlerinin boyutsuzlaştırılması ile, her bir piston açılma genliği için 20, 40 ve 60 bar besleme basınçlarında elde edilen deney sonuçları arasında iyi bir korelasyon bulunduğu gösterilmiştir. Ayrıca her bir valf in 250, 500 ve 750 um piston açılma genliklerinde elde edilmiş olan deney sonuçları arasında da çalışma frekansına bağlı olarak bir korelasyon olduğu görül müştür. Böylece, yapılan çalışmalar sonunda, her üç tip valfde elde edilen deney sonuçlarının, boyutsuz çalışma kuvveti olarak tanımlanan p ` y T 0.4 ?~~s * * - A_ X = Ad O 2 P wx i X, sol maks ile iyi bir korelasyon sağladığı tespit edilmiştir.

Submitted to the Institute of Science Technical University of Istanbul for the Degree of Doctor by Mustafa İNAL In designing a hydraulic control valve for servo systems, it is important to know about the necessary forces to operate the piston. We can classify forces appearing on the spool such as inertia, flow and friction under the dynamic conditions. Although flow forces constitute the main part of the operating forces in hydraulic control valve effectively, it is shown that the friction forces can take the considerable values when the spool is subjected to static conditions, and especially the valve spool and sleeve are not in the ideal cylindrical form or in case where contaminant particles in the oil is larger than the radial clearance. For the reasons stated above, the forces on the spool must be determined and investigated under various conditions.- VI The aim of this work is the study of forces on control valve spools appearing under various operating conditions. Three types of valves were used, critically centered, open centered and closed centered types, each with a nominal diameter of 15 mm, built to narrow tolerances. A multi function special purpose experimental set up was prepared to study the operating forces on the spools of these valves and the experiments were carried out under both static and dynamic conditions. The manufactured valves were simulated with a digital computer all of the possibly significant physical charac teristics in the FWM West German firm. According to simu lation results obtained, each of the three type valves have a stable structure and valves reach their final stable con ditions in about 20 ms. after displacing the valve from its central position and then releasing it mathematically. Prior to force measurements, the flow characteristics of valves were measured and it was determined that they had linear flow characteristics within a spool displacement of ± 500 um. A large proportion of the experiments was done within this region. It was noticed, in addition, that valve discharge coefficients were not constant within this region, therefore, discharge coefficients were found as a function of spool displacement and Reynolds number, at 40 and 60 bar supply pressures. The three types of produced valves stated previously were tested to obtain their pressure sensitivity. Depending upon the results of the test, the following remarkable con-- VII - elusions were obtained. The pressure gain of critically cen tered valve was very high whereas. for open and closed cen tered valves, gains were low due to underlap and overlap conditions. During the leakage tests of the critically centered valve, we observed that the relationship between center leakage flow and supply pressure was not linear. The non- linearity arises from port edges which do not have the ideal sharpness. We car* say that the loss of sharpness is caused ky dirt of oil and production errors. Dynamic and static friction forces were studied in unpressurized conditions and at 20, 40 and 60 bar supply pressures. As a result of these studies, it was noticed that radial forces appeared because of the oval shape of spool lands which was due to production errors, and also because of port effects, both of which were not studied by other researchers before. As a result of these effects, it was determined that Coulomb type friction forces were being created during the dynamic operation, and that these forces were becoming larger when the spool was at rest. This way, the static break-out forces in all three valves were determined while the spools were being pulled 1 ym per minute by a special mechanism developed for this purpose. It was found that the ratio of the static break out forces to the Coulomb friction forces at the end of the break-out was at out (3,5) for closed-centered valves, and (4) for critical ly and open centered valves. Furthermore, it- VIII - was determined, that both the static break-out and dynamic friction forces were proportional to the supply pressure, Steady-state flow forces in valves were also measured and it- was sesi^that, also depending on the spool displace ment, these fc£cc-s rfere always a certain amount larger than the theoretically predicted values. It was understood that this was due tşs?the fact that the Jet flow angle is smaller than its theoretical value, because the orifice edges do not have the ideal sharpness. A detailed study was done to determine the operating forces created during sinusoidal excitation of the spools. First, the spool forces under dynamic conditions were mo delled mathematically as d X ' dX The experimental results indicated an agreement with this model in the low frequency region, but at higher fre quencies, particularly with closed and critically centered valves, a decrease in forces was observed, in addition, the experiments showed that this decrease in operating forces is proportional to the supply pressure and to the spool dis placement amplitudes. The J/ dimensionless operating force, obtained by non- dimensioning the operating forces, was plotted agains-; the- - IX - operating frequency, and a good correlation was observed for the three types of valves. In this correlation, the spool amplitudes were taken as a running parameter and taken the values of 250§lf 500, 750 um. To generalize the correlation, the dimensionjytss operating force T = J 2 AS Ps W XQ 0.4 X maks was defined and a good correlation was obtained which in cludes the spool amplitudes. Furthermore, as a result of the study of these correlation curves, it was observed that these curves can be closely approximated by the frequency response equation of a time constant element with a break frequency of 40 Hz, r. 0,43 1 + 'to'2