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Compressor : Hitachi


Why do problems happen?

No matter how good the equipment may be to start with, running the equipment every day causes its performance to decline gradually and parts become worn.
Once a compressor goes into service, it continues running 24 hours a day, 365 days a year. That means it's running for over 8,000 hours a year. If you were to convert that into a car driving at 60 km/h, that would come to 480,000 km a year.
Before any car drove that sort of distance, there is no question that it would undergo inspection, its mechanical parts would be fully checked out, and anything wrong with the car would be repaired. The same goes for compressors. The longer a compressor runs, the more likely it is that it will run into problems.

Recommendations for periodic inspections

On all machines, parts wear out and deteriorate. Naturally, it is important to prevent accidents, and nip problems in the bud so that the equipment always runs in the best possible condition and has a long service life. This is the goal of all engineers in charge of machinery.
That is why it is extremely important that periodic inspections be conducted.

Information about periodic inspections

Compressors will not necessarily break down even if you go a little bit past the recommended overhaul period. However, breakdown rates will increase as equipment is run over the long term, so we strongly suggest that you conduct planned maintenance and look into future overhauls.

Daily inspections


In daily inspections, operation records are maintained through periodic patrols. By maintaining these records over the long haul, you begin to learn about the specific characteristics of a piece of equipment.

Managing operations

(1) Please conduct periodic maintenance patrols for compressors in operation, paying attention to sound during rotation, vibration, pressure, temperature, and other factors, measure each item, and keep records.

a. Date i. Start, stop time
b. Time j. Other notes (sound, vibration, etc.)
c. Main electric motor voltage k. Oil tank level
d. Main electric motor current l. Drain condition
e. Air pressure m. Oil temperature
f. Discharge temperature n. Y-shaped strainer
g. Coolant water temperature o. Appearance
h. Oil pressure    

(2) Numbers for discharge pressure and temperature should be according to the protector equipment. Rotor contact breakdowns can occur if compressors are run with numbers exceeding those levels.

(3) Make sure that lubricating oil pressure is according to the following conditions.
  a. Oil used: See the specified lubricating oil.
  b. Oil pressure: 0.15-0.22 MPaG (1.5-2.2 kg/cm²G)

(4) Consult the motor drawings or the motor oiling nameplate for oil supplied to the main motor bearings.

(5) During patrols, always make sure that there are no abnormal noises coming from compressors, and gear reducers, motors, etc. If you find an abnormal noise of unknown cause, stop the compressor immediately after you assess the situation.

Daily Inspection Items

Perform daily inspections for the following items.

Product name Inspection items Notes
Filter signal Check the areas highlighted in yellow The intake filter requires maintenance when the front display is red.
Oil tank Check oil level If it is in the “L” range, add oil to bring it to between the “L” and “H” levels.
Drain pipe Check drain condition *Check the drain condition. The equipment may become damaged if it continues to be run with inadequate drainage.

•Be sure to open the drain valve at least once a day to check the drainage condition of the drain.

If there is a significant amount of drainage from the drain valve, the drain orifice may be clogged or the drain strainer may be dirty. In these cases, immediately remove the drain silencer (housed in the orifice) and the strainer, and inspect and clean. Especially during the rainy season when there is more drainage, increase your daily inspections (2-3 times/day).

* Check drainage from the drain. The equipment may become damaged if it continues to be run with inadequate drainage.

Inspections of Auxiliary Equipment

Overhaul cycles

Time 8,000 hours (NH series: 10,000 hours)
Period 1.0-1.5 years (NH series: 2 years)
Description Dismantle, clean, and replace parts for capacity adjusters, air cooler, oil filter, oil pump, and oil filter
  • *Perform this maintenance when the maintenance period or operating time is reached, whichever is sooner.

Description of inspections

1. Intake filter Replace element
2. Intake butterfly valve Dismantle, clean, rust-proof, and replace parts, etc. for valves and hydraulic cylinders
3. Air cooler Dismantle, jet clean, rust-proof, pressure test, replace parts, etc.
4. Oil cooler unit Dismantle the oil cooler/oil filter, replace the oil pump seals, keys, etc.
5. Discharge silencer Dismantle and clean
6. Lubricating oil Total replacement
7. Nylon tube Replace
8. Air block casing Clean out cooling water flow paths
9. Y-type strainer for drain Dismantle, clean, rust-proof, etc.
10. Blowoff silencer Dismantle and clean
11. Apply grease to main electric motor bearings  
12. Electrical panel Visually inspect relays, switches etc., and blow clean with compressed air
  • *We inspect and maintenance other parts (air block, main electric motor, etc.).

Overall Inspection

Overhaul cycles

Series H series NH series EH series SH series UH series
Time 35,000 hours 50,000 hours - - -
Period 4 years 6 years 6 years 6 years 6 years
Description <Description of auxiliary equipment inspections> Dismantle and clean capacity adjusters, air cooler, oil cooler, air filter, oil pump, discharge check valve, and oil filter (replace the secondary element in the NH series and higher)
<Overall inspections> Clean and replace parts for the main unit air block, and clean, insulate, and replace parts for the motor
  • *Perform this maintenance when the maintenance period or operating time is reached, whichever is sooner.

Description of inspections

  1. Air block (special hydraulic tools are needed for all of the following)
    1. Dismantle, clean, apply rust-proof coating, and visually/dimensionally inspect gears, casings, bearings, shaft sealing equipment, rotors, etc.
    2. Replace bearings, O-rings, carbon rings, gaskets, and specified parts
    3. Adjust rotor timing and gaps
  2. Main electric motor
    1. Dismantle rotor and bearing and steam-clean coil and stator coil
    2. Inspect coil binding, apply and dry finishing varnish on rotor surface, replace parts, standalone no-load run test (use insulation tester)
  3. Oil pump motor
    1. Dismantle rotors and bearings, replace bearings
  • Inspect overall and auxiliary equipment, and remove dust on the air side and remove scale on the cooling water side.

Main Electric Motor Inspection


The standard main motor specifications for Hitachi dry screw compressors (NDS-series, SDS-series) are for three-phase induction motors (2-pole and 4-pole motors), which are the self-ventilated open type with greased roller bearings. Dust can clog the air paths and stick to coil surfaces, which causes overheating that leads to burnout. Also, since these units rotate at higher speeds than reciprocating compressors (10-pole and 12-pole motors), the roller bearings can fail if the specified grease is not applied (Hitachi WR-2 grease), which can result in wear damage to the stator and rotor.

Motors need to be overhauled periodically to prevent these kinds of breakdowns and ensure stable operations. Therefore, in addition to daily maintenance, these units should be dismantled and maintenance should be performed on them once every 4 years.

Image: Main electric motor inspection flowchart
Main Electric Motor Inspection Flowchart

Preventative Maintenance

To prevent the main electric motor from overheating, the air inlet filter must be cleaned periodically and grease must be applied periodically to the load side and the non-load side of the bearings while the equipment is running, according to the instructions on the nameplate. Attention must be paid to the exhaust temperature, noise, bearing vibrations, etc. on a daily basis.

Dismantling Maintenance

At the same time as maintenance performed on the SDS main unit every 4 years, we recommend that you dismantle and maintain the motor after 4 years have passed. Dust buildup on the coils and in the airways reduces the effectiveness of cooling. The bearings can be damaged if the grease in the bearing cases deteriorates. We recommend that you dismantle the unit, remove dust, and replace the bearings early, before problems occur.

Quality Assurance

We maintain equipment history records so that we know which parts will need replacement in the next overhaul and what sort of maintenance needs to be performed on the unit. From the standpoint of total quality assurance for compressors as well, we ask that you have us perform overhauls.

High-Voltage Startup Panel Inspections


The user manuals for these compressors ask that our customers perform visual inspections of electromagnetic contactors in the panel and check relays and timers for abnormalities, unusual odors, discoloration, loose screws, etc. Since compressors and high-voltage panels are packaged products, and it is easy to overlook inspection and maintenance tasks for high-voltage panels.

You've recently been experiencing problems with current limiting fuses due to deposits on the contact points of high-voltage electromagnetic contactors and there have been incidents where it has been necessary to replace high-voltage electromagnetic contactors. These incidents could have been prevented if periodic inspections of high-voltage electromagnetic contactors in high-voltage panels had been conducted.

To ensure the safety of equipment and personnel and prevent accidents, it is essential that periodic general inspections be performed for high-voltage panels in addition to the normal inspections.

Specialized training is needed to perform periodic inspections of the high-voltage panel, so please ask our sales office or a Hitachi distributor about having our service personnel perform inspections and repairs.

Inspection and maintenance inside high-voltage panel
Machine name Areas to be maintenanced
Panel unit Dirt and debris within the panel
High-voltage electromagnetic contactors Operating mechanism
Circuit breaker area (arc-extinguishing chamber)
Contact areas, clamped areas
Control devices
Auxiliary contact
Disconnecting switch Main circuit contact area Insulated area
High-voltage limiting fuse Fuse link (fuse element) Fuse holder (support insulator, contact area hardware)
Startup reactor Startup transformer Conductor connector area
Insulated surface (epoxy resin mold)
Case deformation (compound-filled)
Power cable Main cable unit (coated)
Terminal area
Protective relay Output contact points
echanical parts
Coil, electrolytic condenser, photocoupler
Auxiliary relay
Time limit relay
Output contact points
Coil, coil terminals
Case terminal block, terminals
Time limiting setting value
Operating and control circuits terminal block
Opener, secondary terminal, auxiliary contactor terminals
Clamping terminal
Insulating material between terminals
Contactor area
Indicators Indicator
Operating, switch opener Handle, installation seat
Terminal, terminal block
Wiring circuit breaker Contact terminal
Open-close operation
High-voltage PT fuse Fuse link (fuse element)
Fuse holder (magnetic)
Control fuse Fuse link (fuse element)
Fuse holder
Control wiring Wire insulation
Terminal area
Signal lamp -

Million-cycle unloading inspection


The unloading control method for SDS compressor capacity control is performed in steps of 0-100%. The following diagram describes the unloading mechanism, which consists of electrical and mechanical parts. Please perform maintenance and replace parts based on the number of operations and years of service.

Replacement standards for unloading electrical parts
  Number of operations Years of service Notes
SDS-H series 1 million cycles About 4.0-6.0 years 500,000-cycle inspection
(Replace as needed)
SDS-NH series
SDS-EH series Auxiliary relay replaced at 500,000 cycles. Otherwise, inspect
(Replace as needed)
SDS-SH series
SDS-UH series

Unloading mechanism
Image: Unloading mechanism chart

  • *The above reference chart is based on the SDS H and NH series (relay panels).
Electrical parts subject to replacement and examples of breakdowns
Part name Symbol Problem
Four-way solenoid valve 20V1 Stuck spool (inoperable)
Discharge voltage switch 63A3 Pressure setting deviations (anomalous oscillations)
Auxiliary relay 20VX Rough contact points (continuous unloading)
Unloading counter HC Cycles not counted (cannot manage number of duty cycles)
Blowoff pressure switch 63A1 Pressure setting deviations (protector circuit malfunction)
Blowoff pressure timer 62A1 Limit counter malfunction (protector circuit does not function)
  • *There is a higher likelihood of these kinds of malfunctions, so we recommend that you be proactive about replacing parts (keep spares on hand).
  • *These examples of parts configurations are modeled on the relay panel. Symbols and specifications for the E-panel (electronic panel) and N.I.C.S. panel provided may differ. Please contact us if you have any questions.