Repair and Service for Hydraulic Systems

The Bucyrus DBT Europe company from Lünen engaged Montanhydraulik Repair and Service with the repair of a test cylinder which is an important component of a test machine for hydraulic rams for mining.

The results of the dismantling and analysis were that the cylinder’s rod guide was seized in the guide bushing. To correct this problem, the rod guide was rebuilt as hard fabric guide rings. In addition, the rod was completely remanufactured. Delivery took place in 2009.

Hydraulic cylinder:

• Compression force: 15,000 KN = 1,500 t
• Weight: 31 t
• Piston diameter: 900 mm
• Rod diameter: 500 mm
• Stroke: 1,600 mm

It was the first repair of the steering cylinder since the BG 285 bucket wheel excavator was built in 1975. Wear, combined with enormous impact forces, had completely destroyed the piston sealing system and the sealing material. The axial holding rings for the piston seal had come away from the piston and were jammed in the cylinder bore. Because of the serious leakage through the disintegrated piston sealing system, the forces required to move the steering shaft could no longer be applied. The digger could not be manoeuvred.
Because Montanhydraulik Repair and Service had already successfully repaired two similar cylinders in the past, the repair company was also engaged to repair this steering cylinder. For this, the greatest challenge was the piston rod head. Initially shrunk onto the rod through heating and cooling, in order to transfer tensile forces of 10,000 kN, it had to be removed in the same way, so as to be able to dismantle the cylinder weighing 45 t. In fact three attempts were necessary, requiring ten hours of patience and a certain amount of technical creativity, until the six ton rod head could be loosened with the aid of hydraulic pressure and by heating the rod head to 300 ºC

Good collaboration with Montanhydraulik in Holzwickede

It was then clear that the 610 mm length and 500 mm fitting diameter of the two parts were heavily grooved, and that there were numerous deep seizure marks. However, through the support of Montanhydraulik in Holzwickede, the repair lasted only one day longer than planned. For the necessary turning tasks, the technicians first built a steady-rest which was freely adjustable on the piston rod, and reduced the diameter of the rod by 4 mm. They then welded the reduced diameter up to size using submerged arc welding, and machined it again to the true diameter. The rod head was machined to +0.5 mm. The work required the greatest precision, because the press fit had to be created as per the original pattern, so that the tensile forces could again be fully transferred without any problems.

Vertical cylinder installation

The final obstacle was to shrink the rod head back onto the piston rod in such a way that the press fit could be used to its full extent and that the repaired steering cylinder would guarantee the problem-free operation of the bucket wheel excavator. For the installation of the cylinder, the piston rod with piston and cylinder head were suspended from a 300 t mobile crane hook lowered through the opened roof of the manufacturing hall. Beneath this was the vertically aligned rod head, mounted in a steel frame; this was again heated to 300 ºC, while the fitting diameter of the rod was cooled to –150 ºC using liquid nitrogen. The piston rod was then lowered little by little into the rod head. After successful function and pressure testing, as well as filling with 3,500 litres of hydraulic oil, the steering cylinder was finally transported back to the customer on a special low-loader – punctually and maintained to be as good as new.

These requirements were successfully fulfilled:

• Maximum repair time of 3.5 weeks
• Installation capacity suitable for cylinder weight and size
• Manufacturing capacity for replacement parts having 1,000 mm diameter
• Replacement and optimisation of the sealing and guiding elements
• Renewal of the fit between piston rod and rod head
• Refurbishing and repolishing of the chroming on the piston rod
• New sealing of the cylinder, even when dimensions changed because of repairs
• New manufacture of six holding rings for the piston seal (ø 1,000 mm), a seal for the guide bushing, and the bronze piston (ø 1,000 mm)
• Honing the cylinder bore
• Inspection and repair of the valves fitted to the cylinder
• Installation of valve connection plates and adjustment of the pipework to implement the installation locations of the valve blocks
• Pressure testing and filling with hydraulic oil
• Renewal of the paintwork on the cylinder as per customer requirements for corrosion protection coatings, including sand blasting

Technical data for hydraulic cylinder:

• Weight: 45 t
• Piston diameter: 1,000 mm
• Rod diameter: 500 mm
• Total stroke: 4,400 mm
• Oil capacity approx. 3,500 l

Technical data for bucket wheel excavator:

• Year built: 1975
• Service weight: 13,500 t
• Rated supply capacity: 200,000 m3/day
• Length: 210 m
• Height: 92 m
• Chassis width: 45 m
• Number of crawlers: 12
• Drive power: 16,560 kW

The opencast mines of RWE-Power AG in Hambach and Gartzweiler extract the largest amounts of brown coal in the world, around 100 million t per year. For this, the BG 290 bucket wheel excavator shifts some of the enormous amounts of overburden, and its hydraulic steering ensures that the huge piece of equipment navigates through the terrain with precision. Only one of the three tracks upon which the machine stands is controlled during travel, through the steering shaft powered by a giant hydraulic cylinder.

Because of the loosened stones which continuously fall onto the semi-extended piston rod during the operation of the excavator, at some stage the chrome surface had become littered with impact marks. Because of this, RWE appointed Montanhydraulik Repair and Service to repair the heavyweight colossus.

These requirements were successfully fulfilled:

• Maximum repair time of 25 days
• Installation capacity suitable for the weight and size of the cylinder
• Manufacturing capacity for replacement parts having 1,000 mm diameter
• Replacement and optimisation of the sealing and guiding elements
• Design and construction of a protective cover against stone impact
• Repair and recoating of the piston rod using a chrome/nickel layer as protection against wear and corrosion
• Resealing of the cylinder specifically for the changed dimensions resulting from the repair
• Replacement of the pivoting bearings
• Honing the cylinder bore
• Inspection and repair of the valves fitted to the cylinder
• Pressure testing and filling with hydraulic oil
• Renewal of the paintwork on the cylinder as the RWE guidelines for corrosion protection coatings, including sand blasting

Technical data for hydraulic cylinder:

• Weight: 30 t
• Piston diameter: 1,000 mm
• Rod diameter: 500 mm
• Total stroke: 4,400 mm

The infeed and outfeed cylinders of a roller cement mill are in use right round the clock. That is why serious leakages occur on the piston rod sealing, even after only three or four months. Montanhydraulik Repair and Service developed a solution which contributes to quadrupling the service life of the cylinders.

Powered by a 2,000 kW electric motor, the milling drum conveys cement clinker, slag, or limestone from the neighbouring quarry under the milling roller. During the milling process, the cylinder seals must withstand short-stroke and rapid swerving motions which have high pressure peaks; these are caused by the stones in the milled material. On top of this, the various milled materials are milled at different levels of high pressure, and this results in temperatures up to 100 °C and significantly reduces the service life of the seals. In difficult operating circumstances, correspondingly high leakage rates set in after only a few weeks – a situation that the operator wanted to change.

The following steps were taken as part of the cylinder repair: Instead of steel, Montanhydraulik Repair and Service created a new seal holder bushing made of bronze, which has a smaller gap size in the triple seal system arranged in cascade-form. The fitting tolerance of the new guide bushings made of bronze was also reduced. For this, the bushing was installed into the cylinder head using a press fitting (shrunk-in using liquid nitrogen). After this, the play of the rod guide was adjusted exactly to the toleranced diameter of the rod, using finish turning.

In order to fulfil the new safety requirements of the German Federal Railway Authority, a German railway company had to install a safety circuit into its bulk material wagons in order to upgrade the existing hydraulics for actuating the wagon discharge flaps. At the same time, the manual actuation was to be converted to hydraulic actuation by means of pilot valves. The contract for the installation of two sample systems was awarded to Montanhydraulik Repair and Service in Dortmund.

Together with the development department of Montanhydraulik, the repair company developed a new control block unit for the hydraulic actuation, delivered as an installation kit, and a safety circuit which can be built into the existing system.

After certification and the successful test phase of the sample wagons, Montanhydraulik Repair and Service received the go-ahead for the conversion of the hydraulic aggregates of 136 railway wagons. The first converted systems were delivered at the start of 2011.

These requirements were successfully fulfilled:

• Creation of a performance specification for the conversion and retrofitting of the systems
• Creation of a functional description and complete design drawings and circuit diagrams for the conversion
• Safety inspection by FMEA
• Construction of two new sample systems having a new control block unit
• System optimisation
• Conversion of 136 bulk material wagons

Dillinger Hütte provides its customers, e.g. power stations, with components made of heavy plate, which they cannot manufacture themselves. The components include edge-machined plates, spherical segments, single or multi-component tank bases, as well as pressed parts. The latter are formed with the aid of multipurpose presses or so-called dish presses.

At Dillinger Hütte, after a multipurpose press having a total press force of 2,500 t had last been overhauled in the 1990s, signs of wear increased alarmingly in recent days. After a noteworthy running time exceeding 20 years of multi-shift operation, the piston rods were showing signs of grooving, and the seals were displaying signs of significant wear.

Because the repair could not take longer than two weeks, Dillinger Hütte placed the order with the Dortmund company Montanhydraulik Repair and Service, as it has the technical know-how for cylinders of this size, and in terms of manufacturing technology it is fully equipped for these weights and dimensions.

These requirements were successfully fulfilled:

• Grinding and hardening the piston rod of the central ram
• Coating the clamping cylinders with SuperCoat
• Hard chroming the piston rods of the plate holding and hold-down cylinders
• Honing all cylinder bores
• Renewal of all bronze guide bushings
• Fitting new seals for rods and pistons
• Conversion of the piston guides and seals to a Merkel slide ring system
• Grinding the leading external diameter of the cylinder bore of the central ram
• Manufacturing new guide bushings having a diameter of 1,250 mm, adjusted to the ground external diameter of the central ram
• Vertical installation and hydraulic pressure and function testing in a specially manufactured holding device
• Return delivery of the central ram, using special transport requiring specific approval

Type and quantity of the repaired cylinders:

• 1 central ram cylinder having a press force of 2,500 t
• 4 clamping cylinders
• 8 hold-down and plate holder cylinders, each having a press force of 500 t

Technical data for the central ram cylinder:

• Piston diameter: 870 mm
• Piston rod diameter: 710 mm
• External diameter of cylinder bore: 1,200 mm
• Stroke: 3,500 mm
• Weight: approx. 36 t
• Operating pressure: 250 bar

After only a brief period of operation, the tipping cylinder of an electric arc furnace at Georgsmarienhütte broke down because of leakage on the piston rod. Damage analysis by the service technicians of Montanhydraulik Repair and Service resulted in the following findings: Electrostatic charges had caused functional impairments and crater-formed damage to the hard-chromed piston rod.

In addition, the hard chrome layer had been attacked by electro-chemical corrosion. Finally, the large amount of dust arising resulted in rapid abrasive wear to the rod seals.

The onsite problem analysis of the dismantled tipping cylinder resulted in conversion of the cylinder to a completely isolated piston and rod guide, continuous rod flushing, and a fused bonding coating of the piston rod. This bundle of measures extended the service life from six months to four years.

Furnace tipping cylinder:

• Piston diameter: 320 mm
• Rod diameter: 220 mm
• Stroke: 2,600 mm
• Operating pressure: 230 bar

Chipboard manufacture takes place in a press system which operates continuously. The press force is built up using hundreds of press cylinders arranged one after the other in series. During operation, deformation of the cylinder can arise, and this very often results in leakages on the static seal of the threaded clamping ring.

On the basis of the damage analysis conducted as part of the repairs, Montanhydraulik Reparatur und Service GmbH performed a conversion of the press cylinder to a flanged clamping ring, in order to transfer the position of the static seal to an area where there was less deformation.

Further benefits of the conversion measures are the guarantee of the sealing effect even during significant deformation under load, and the simple and rapid removal of the clamping ring in the event of a repair.

Press cylinder:

• Piston diameter: 327.5 mm
• Rod diameter: 300 mm
• Stroke: 110 mm