By Timothy McCrea, Market Segment Manager, Aftermarket Hägglunds, Bosch Rexroth Corp.
Hydraulic drive systems are heavily used in pulp mill and wood yard operations for critical equipment such as twin roll presses and high-pressure feeders, digester outlet devices, and log decks. Radial piston direct drive hydraulic motors are well suited for these applications because of their low-speed/high-torque capabilities, space-saving design, and well-established record for maximum uptime and operating efficiency, particularly for operations in pulp mills requiring near round-the-clock performance.
As with any system in a demanding environment, routine hydraulic service is critical to maintaining uptime. To reap the full benefit and potential of direct drive hydraulic systems, there are several maintenance best practices and techniques to follow. These include proper initial fluid selection, routine fluid analysis and troubleshooting, evaluating and replacing internal components, and condition monitoring.
Hydraulic direct drives are engineered for longevity; but preventive maintenance is often overlooked for this type of equipment until it is too late. As with any system in a rugged environment, routine service is critical to maintaining the uptime of radial piston motors. When breakdowns occur, they can take valuable equipment off the line or even shut down operations.
Implementing a few effective maintenance and service best practices can prevent major breakdowns, plus it saves time, money, and effort compared to allowing equipment to deteriorate, then shipping it off to a repair center. More importantly, these best practices can help pulp and paper manufacturers ensure that the drives powering vital equipment provide the years of operation they were engineered to deliver.
Large, low-speed/high-torque hydraulic direct drive systems are often used to power demanding pulp and paper manufacturing operations in which heavy masses of materials are moved under variable speeds. Whether it’s lifting or lowering loads at a steady speed, executing straight-line or rotating movements, generating steady accelerations and maintaining set speeds, or efficiently transferring power, hydraulic direct drives are proven to handle the operating conditions in harsh pulp mill environments as well as handle shock loading, two of the most common operating conditions.
Low-speed/high-torque radial piston motors meet these challenges, producing extremely high torques in a compact package. The radial piston motor mounts directly on the machine’s drive shaft, while other hydraulic drive components can be located away from the machine.
Some of the better radial piston hydraulic drives are designed to prevent equipment damage via set points and limits that prevent overtorque in case of upstream or downstream equipment failure. Diagnostics and machine health indicators are additional advantages available on these types of radial piston hydraulic drive systems. Given the central role hydraulic direct drives play in the smooth operation of pulp mill and papermaking equipment, proper care for their operating condition should be included in any effective maintenance strategy.
When periodic predictive maintenance practices are performed intermittently or not at all, even the most well-engineered hydraulic system breaks down. This “breakdown maintenance” can have serious impacts, like these:
In addition, if a facility or location practices breakdown maintenance, failures in multiple pieces of equipment can happen on top of each other. These types of failures typically have longer repair times if systematic neglect forces repair of surrounding components and equipment as well as the hydraulic direct drives.
Several essential maintenance practices can prevent potential hydraulic direct drive failures and keep you from operating in breakdown maintenance mode.
Periodic inspection and maintenance checks. Periodic inspection lays the foundation for maintenance best practices. Equipment inspections should follow a standard schedule. Standard intervals should be monthly at a minimum, whereas some of the best facilities conduct weekly inspections. Inspections should document danger signs, such as main pump pressure changes, fluctuations in desired motor speed, increased oil temperatures compared to standard operating levels, increased case drain flows, hydraulic oil leaks, and oil reservoir levels.
Large-scale operations such as pulp and paper mills use these inspections to do more than prevent breakdowns. They also capture data about machine performance to improve their maintenance and production processes.
Hydraulic fluid selection and condition. As with other hydraulic systems, be sure to use the manufacturer’s recommended fluid with the viscosity for which the hydraulic direct drive is engineered. Environmental factors can influence this selection. Pulp mill operations can experience temperature extremes based on their locations, especially in high-temperature environments. If equipment manufactured for use in Canada is used in South Carolina or Georgia, doublecheck the hydraulic system specifications as to whether a change from a moderate viscosity fluid, such as ISO 46, to a denser viscosity, such as ISO 68 or 100, is advisable.
Equally important is regular monitoring of hydraulic fluid conditions for contaminants, discoloration, or particles, along with other sensor data the direct drive may offer. Oil condition is a critical warning sign that there may be components in the motor that are wearing or damaged.
Servicing filters and seals. Filter changes at specific intervals are important to reduce contamination buildup and, in turn, reduce wear on all hydraulic system components. Direct drive manufacturers specify the type of filter their system should use and may provide more detailed recommendations – filter medium and micron level, for example – depending on the level of dust and contaminants present in the operating environment.
In addition, most hydraulic systems have fittings that use O-rings to prevent leakage. However, due to shock loading, vibration, and other factors, these O-ring seals may tend to wear out more frequently. Periodic maintenance inspection finds those leaks so you can replace used and worn O-rings, tighten loose fittings, or replace any damaged ones.
Use OEM parts. With radial piston motors, it may be assumed that a local repair shop has the expertise to complete a repair to OEM specifications. However, to complete a repair to a “like new” condition, it is critical to use the right replacement parts.
It’s quite common to see motors repaired with used parts taken from a scrapped component, or with aftermarket parts not necessarily designed to perform under the harsh conditions of high shock load equipment. In addition, sometimes motors have parts that require a special proprietary coating that may improve service life or efficiency. These coatings are utilized on internal wear parts designed specifically for high shock load applications, where there is a risk of cavitation. Cavitation can cause a reduction of oil film between the metal wear parts.
In addition, these proprietary coatings were developed to deliver long life in applications requiring extremely slow and extremely high motor speeds where oil viscosity levels can be reduced. This detailed knowledge may not be available from local third-party repair shops that work on a wide range of industrial hydraulics equipment from multiple manufacturers.
It’s important to specify that any repairs should use original OEM parts supplied from the manufacturer and not those pulled from scrapped equipment. In fact, some hydraulic direct drive manufacturers, such as the Hägglunds group from Bosch Rexroth, do more than repair their equipment; they can provide fully remanufactured drives and radial piston motors with “like new” warranties to ensure long-term performance.
Choose OEM-certified suppliers. The specialized design and operating characteristics of hydraulic direct drives call for a higher level of expertise and resources, which is typically supplied exclusively by OEM-certified facilities that concentrate on this equipment. Third-party repair facilities will not have the original manufacturer specifications to properly repair, calibrate, and test the motor to new condition – and they will not typically perform a fully warrantied remanufacture.
OEM-certified service suppliers also provide the most effective and up-to-date preventive maintenance programs, with factory-trained technicians to inspect, service, and maintain the OEM’s equipment. Certain proprietary components that cannot be duplicated or obtained from scrapped drives and motors may only be available from OEM-certified facilities. There also may be specific equipment tolerances that only the factory or OEM service shop is equipped to work with.
Preventive maintenance programs. Many of the practices we’ve discussed can be incorporated into comprehensive preventive maintenance programs that identify risks and correct issues before they lead to failures or downtimes. Effective PM programs include annual major inspections and quarterly minor ones carried out in the field by factory-trained and certified technicians.
Good PM programs include detailed inspections of all accessible equipment, oil temperature, pressure and nonintrusive motor temperature inspections, pump settings, speed and motor parameter setting inspections, and validation against operating specifications, case drain flow measurement, and oil analysis, as well as shaft coupling and O-ring inspection and replacement. An effective PM program will also supply a detailed report of findings, recommended maintenance, spares, and follow-up actions.
The goal of truly effective maintenance and service programs is to significantly reduce downtime, improve safety, and ensure hydraulic direct drives maximize their operational value. Hydraulic direct drives are engineered to provide years of service, even in demanding pulp and wood yard operations, which is why proper maintenance programs from service facilities operated by or authorized by the equipment’s OEM make the most sense in the long run.
OEM facilities have the technical personnel, original specifications, direct access to manufacturer’s parts, and the field experience necessary to keep hydraulic direct drives operating smoothly through their full life cycle. In addition, they can advise pulp and paper plant operators when it makes sense to retire legacy equipment once it becomes cost prohibitive to maintain. They can help develop a cost-effective direct drive upgrade or migration plan based on the unique needs of specific locations, operations, and production equipment. That is one more way to leverage the maximum value from partnering with experts to extend the life of hydraulic direct drive systems.