p u m p s su sp en d ed in th e oil tan k, or th ey are m ou n ted on su p p orts in th e p ress fou n d ation . Th e m otors d rive th e p u m p s via an elastic clu tch . Th e
p u m p s are su p p lied by su ction p ip es, an d th e h yd rau lic flu id is con veyed to th e valve con trol by m ean s of h igh -p ressu re p ip es. Th e valve control
system gu id es th e h yd rau lic flu id to th e ap p rop riate u n its. Dep en d in g on th e fu n ction in q u estion , th e h yd rau lic cylin d ers are exten d ed or retract-
ed , m oved actively or p assively in th e p ress. Th e retu rn flow is gu id ed back to th e oil tan k. Th e consum ers, cylin d ers or h yd rau lic m otors, m ove
th e slid e or cu sh ion s, or ad ju st sp in d les. Th ey gen erate forces, lift load s or clam p th e d ies. Th e service unit op erates in a byp ass circu it away from th e
m ain h yd rau lic flow. In th is u n it, th e flu id is recircu lated by m ean s of fil- ters an d coolers in a sep arate circu it, p u rifyin g an d coolin g it to en su re
th at th e p ress rem ain s read y for p rod u ction at all tim es. Pressure accum ulator drive system
If t h e p u m p co n veys t h e flu id t o a p ressu re accu m u lat o r, it is p o ssible t o sm o o t h o u t t h e en ergy d elivery fro m t h e m ain elect rical n et . Large
q u an t it ies o f h yd rau lic o il can be d rawn fro m t h e p ressu re accu m u lat o r fo r a sh o rt t im e, wh ich h as t o be rep len ish ed by t h e p u m p befo re t h e
n ext p ress cycle. If t h e ext ract io n o f h yd rau lic o il is fo llo wed by a lo n g id le p erio d , t h e accu m u lat o r can be su p p lied by a sm all p u m p – fo r
exam p le wh en lo ad in g an d u n lo ad in g t h e d ie aft er ret ract io n o f t h e slid e. However, for p rocesses req u irin g large q u an tities of h yd rau lic flu id
to be d rawn in q u ick su ccession , a h igh er d elivery p ower m u st be avail- able – fo r exam p le a d rawin g o p erat io n fo llo wed by t h e o p en in g o f t h e
p ress. Fo r t h e seco n d exam p le, a p ressu re accu m u lat o r-t yp e d rive sys- t em is n o t su it able, sin ce n o t o n ly a large accu m u lat o r syst em bu t also
a large p u m p is req u ired . Draw p resses in vo lve sh o rt cycle an d t ran sp o rt t im es wh ich o ft en can n o t be ach ieved u sin g an accu m u lat o r d rive sys-
t em . Table 3.3.1
d em o n st rat es ad d it io n al d ifferen ces bet ween p u m p an d accu m u lat o r d rive syst em s.
3.3.2 Hydraulic oil
Hyd rau lic syst em s in p resses are o p erat ed u sin g st an d ard h yd rau lic o il HLP acco rd in g t o DIN 51524 Part 2. Hyd rau lic o ils are req u ired t o
co m p ly wit h st rict st an d ard s:
77
Hydraulic presses
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78
Fundamentals of press design
– lu bricat io n an d wear p ro t ect io n , – co n st an t visco sit y level at t em p erat u res bet ween 20 an d 60 °C,
– resist an ce t o t em p erat u re, – lo w co m p ressibilit y,
– lo w fo am in g ch aract erist ics, – lo w abso rp t io n o f air,
– go o d ru st p ro t ect io n , – go o d filt rat io n p ro p ert ies,
– lo w co st . All h yd rau lic u n it s are d esign ed fo r u se wit h h yd rau lic o ils co n fo rm in g
t o t h ese st an d ard s. Ch aract erist ics, life exp ect an cy an d p ressu re ran ges, as well as t h e ch o ice o f seals an d gasket s co rresp o n d t o t h e o il t yp e u sed .
Du e t o t h e com pressibility of hydraulic oil, t h e vo lu m e o f h yd rau lic flu id ch an ges u n d er p ressu re. As p ressu re is in creased , t h e flu id co lu m n is
co m p ressed . Th is p ro p ert y affect s t h e co n t ro l an d regu lat in g fu n ct io n s in t h e p ress: t h e h igh er t h e p ressu re, t h e lo n ger t h e resp o n se t im e.
W h en calcu lat in g cycle t im es o r d et erm in in g t h e u se o f d ifferen t d rive syst em s, t h e co m p ressibilit y fact o r m u st be t aken in t o acco u n t , as t h is
h as a p ro n o u n ced effect , p art icu larly wh ere sh o rt fo rm in g st ro kes are in volved , for exam p le wh en blan kin g or coin in g. In th e case of m in eral
oils, a com p ressibility factor of 0.7 to 0.8 p er 100 bar oil p ressu re can be exp ected . Th is p rop erty of h yd rau lic flu id s also affects th e release of
Table 3.3.1: Comparison of hydraulic drive systems
Pump drive system
The installed pump capacity determines the achievable speeds w ith the motor output.
Requirement-oriented hydraulic oil delivery results in peak loads on the electrical mains.
Work capacity is unlimited, as the pressing force is available over the entire stroke.
The system pressure corresponds directly to the pow er requirement of the die.
The pressing force can be simply limited by means of pressure reducing valves.
Pressure accumulator drive system
Speeds are determined by valve cross-sections and the pressure head.
Continuous hydraulic oil delivery exerts an even degree of stress on the mains.
Work capacity is limited by the available useful volume of the accumulator.
Pump pressure corresponds to the accumulator boost pressure. In case of a low er energy
requirement, excess energy is converted into heat.
It is not possible to regulate the pressure of an accumulator, w hich alw ays operates at full
capacity.
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79
Hydraulic presses
p ressu re in t h e p ress cylin d er. If t h e area u n d er p ressu re is o p en ed t o o rap id ly, t h is resu lt s in p o t en t ially d am agin g p ressu re relief sp ikes.
Flam e-ret ard an t h yd rau lic flu id s sh o u ld o n ly be u sed wh ere t h is is abso lu t ely m an d at o ry an d t h e ap p lian ce m an u fact u rer h as issu ed t h e
relevan t ap p roval. Th ese call for th e u se of sp ecial gaskets, in som e cases lower op eratin g p ressu re an d also sp ecial m easu res d u rin g start u p .
Bet ween t h e p u m p d rive syst em an d t h e cylin d ers, t h e p ress fu n c- t io n s are co n t ro lled by valves. Direct io n al, p ressu re an d n o n -ret u rn
valves act in co n ju n ct io n wit h p ro p o rt io n al an d servo valves. Dep en d in g o n t h e syst em an d p o sit io n – in t h e p rim ary o r seco n d ary
circu it – valves o f vario u s d im en sio n s are u sed . Th e execu t io n o f all t h e n ecessary p ress fu n ct io n s calls fo r a large n u m ber o f h yd rau lic co n n ec-
t io n s wh ich are gro u p ed t o get h er in a blo ck co n figu rat io n Fig. 3.3.4
. Block hydraulic system s h ave gain ed in p o p u larit y, becau se:
– ext ern al co n n ect io n s o n ly n eed t o be d irect ed fro m t h e p u m p an d t o t h e cylin d er,
– t h e valves are su rface-m o u n t ed an d easy t o exch an ge,
Fig. 3.3.4
Block hydraulic system
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
– t h eir n at u re in evit ably resu lt s in a st an d ard izat io n p ro cess: t h e valve blo cks d o n o t n eed t o be red esign ed fo r each d ifferen t p ress,
– all t h e valves are bro u gh t t o get h er t o creat e a fu n ct io n al assem bly o n a sin gle blo ck,
– in t egrat ed valves are sen sibly co m bin ed wit h sp ace-savin g, lo w-co st au xiliary valves,
– t ro u blesh o o t in g is sim p lified by t h e p resen ce o f d rilled -in fu n ct io n al an d t est co n n ect io n s.
3.3.3 Parallelism of the slide
