Marine Engineering Study Materials

Operation of Hydraulic Jacks

In principle, a screw can be tightened when nut is screwed in during tensioning of stud by hydraulic force and then release the hydraulic force. This hydraulic tightening method provides easier working and more reliable result than manual tightening in general. Therefore, most of the important or big screws are tightened hydraulically on ships such as cylinder head nuts, exhaust valve nuts, for crank pin bearing, cross head bearing, main bearing, tie bolts, etc. A hydraulic tightened screw always consists of a set of stud and nut which are designed properly for a hydraulic tool set. The nut has pin holes for turning through the hole of support for jack as shown in figure below.

The hydraulic tool set consists of a hydraulic pump, hoses and a distributing piece, which are for common use regardless of screw size. Most of the hydraulic screws are tightened in pair, which need distributing piece for same tightening pressure. There are two kinds of distributing piece with different number of ports. Screws on cylinder head need four ports and the others which need two ports use the corresponding distributing piece.

Each screw needs own set of hydraulic jack and support for the corresponding screw size. Depending on the working condition, extension screw should be added for the jack as shown in figure below.

Each screw also needs own tightening pressure which should be kept absolutely.

Note: Before pressurizing the hydraulic jack, make sure the pressure gauge is calibrated and display the actual pressure. Applied oil pressure must be within a range of ±5 bar of given pressure for each stud type.

Hydraulic Jack Tightening Procedure

1. Check pre-tightening of the stud and tighten by wrench, if loosened.
2. Clean studs, nuts and around seats.
3. Screw the nut manually until the nut contacting the seat closely.
4. Check condition of the hydraulic tool set and make them ready to use.
5. Mount support (extension screw) and screw the jack by hand.
6. Connect hoses from the pump to the jacks via the distributing piece.
7. Close the release valve of the hydraulic pump.
8. Open air venting plug of the jack and check air venting by pumping.
9. Close the venting plug and pressurize up to the specified pressure for tightening the nut.
10. Turn the nut to be screwed firmly by a pin manually through the hole of the support.
11. Release the hydraulic pressure by opening the release valve of the hydraulic pump.
12. Repeat pressurizing and check the nut loosened.
13. Retighten the nut, if loosened.
14. If the stud or nut has been replaced by new ones, repeat the tightening three times for the first tightening. This repeat is necessary for
    the settlement of the threads.
15. Release the hydraulic pressure and dismount the tool set.

Hydraulic Jack Loosening Procedure

1. Clean seat and threads.
2. Check condition of the hydraulic tool set and make them ready to use.
3. Mount support (extension screw) and jack.
4. Connect hoses from the pump to the jacks via the distributing piece.
5. Turn the piston of jack until contacting the nut and then unscrew the piston by about half turn. This is important to provide space for loosening
   the nut.
6. Close the release valve of the hydraulic pump.
7. Open air venting plug of the jack and check air venting by pumping.
8. Close the venting plug and pressurize up to the specified pressure for loosening the nut.
9. Unscrew the nut by half turn manually by means of a pin through the hole of the support.
10. Be sure to check that loosened nut moves freely without contacting the piston of the jack. Otherwise, jack and the nut may be stuck each other after releasing hydraulic pressure. If stuck, increase hydraulic pressure slightly and turn the nut about 1/4 turn clockwise for loosening from the piston of the jack.
11. Release the hydraulic pressure and dismount the tool set.

Reference:

Himsen Instruction Book by HYUNDAI HEAVY INDUSTRIES CO., LTD. Engine & Machinery Division