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Tips to better manage direct-drive motors

By |  December 30, 2020
Photo: Bosch Rexroth

Maintenance inspections can capture data about machine performance to improve production processes. Photo: Bosch Rexroth

Hydraulic systems are proven, versatile and dependable technologies used in quarry and mining operations on equipment such as auger drives, drilling machines, mobile crushers, apron feeders, breakers, belt conveyors and more.

In particular, radial piston direct-drive hydraulic motors are well-suited for these applications due to their low-speed and high-torque capabilities, their space-saving design and technology engineered to deliver reliable, long-lasting performance, and their reliability in moving heavy loads under difficult operating conditions.

As with any system in a rugged environment, routine service is critical to maintain the uptime of radial piston motors. Hydraulic direct drives are engineered for longevity, but preventive maintenance is often overlooked for this type of equipment until it is too late. When breakdowns occur, they can take valuable equipment offline or even shut down operations.

As with most industrial equipment, implementing a few effective maintenance and service best practices can prevent major breakdowns. More importantly, these best practices can help pit and quarry operators ensure the drives powering vital equipment provide the years of operation they were engineered to deliver.

Value of hydraulic direct drives

Large, low-speed/high-torque hydraulic direct-drive systems are often used to power demanding pit and quarry operations where heavy masses of materials need to be moved under variable speeds.

Quarry equipment such as fixed or mobile crushers, bucket wheel drives and augers have specific needs in equipment design and flexibility. These include the ability to withstand harsh environments and shock loading – two of the most common operating conditions. In addition, hydraulic direct drives must be able to resist wear and tear without adding bulk and weight.

Low-speed/high-torque radial piston motors meet these challenges, producing extremely high torques in a small, compact package. The radial piston motor mounts directly on the machine’s driven shaft, while other hydraulic-drive components can be located away from the machine. For mobile equipment, all the hydraulic components may be distributed on the machine platform itself.

Some of the better radial piston hydraulic drives are designed to prevent equipment damage via set points and limits that prevent over-torque in the event 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 pit and quarry equipment, they should be included in any effective maintenance strategy.

Breakdown maintenance

Photo: Bosch Rexroth

The specialized design and operating characteristics of hydraulic direct drives call for a high level of expertise and resources. Photo: Bosch Rexroth

When periodic predictive maintenance practices are not in place – or only performed intermittently – even the best, most well-engineered hydraulic system will break down. Breakdown maintenance follows with serious impacts:

■ Equipment deterioration is a direct result of neglect, increasing energy costs and reducing efficiency.

■ Small issues, such as fluid leaks from worn-out seals, become big issues when preventable fixes are overlooked.

■ Safety incidents and injuries occur more often due to sudden failures or jams on conveyors and other systems moving loads over long distances.

■ Maintenance teams are under pressure to complete fixes on a time crunch, thus potentially overlooking standard safety practices.

■ Emergency repairs are more difficult to schedule, often incur higher costs and can take technical personnel away from regular duties to support removing and transporting damaged direct-drive equipment.

In addition, if a facility or location generally follows breakdown maintenance, failures in multiple pieces of equipment can happen right 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.

Best maintenance practices

There are several essential maintenance practices that, if followed consistently, can prevent potential hydraulic direct-drive failures while keeping producers from operating in breakdown maintenance mode:

■ Periodic inspection and maintenance checks. This 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 use these inspections not only to prevent breakdowns, but also to capture data about machine performance to improve both 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 level the hydraulic direct drive is engineered to use.

Environmental factors can influence this selection. Aggregate operations will have temperature extremes based on their locations, especially in high-temperature environments. For example, if equipment manufactured for use in Canada is used in New Mexico, double-check the hydraulic system specifications for 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 that 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. This is particularly important in pits and quarries, with their high level of dust, gravel and other contaminants.

■ Servicing filters and seals. Filter changes at specific intervals are important to reduce contamination buildup and, in turn, reduce wear in all hydraulic system components. Direct-drive manufacturers will specify the type of filter their systems should use, and they 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 utilize O-rings to prevent leakage. Still, due to the shock loading, vibration and the nature of leaking in aggregate operations, these O-ring seals may tend to wear out more frequently. Periodic maintenance inspections will find those leaks so producers can replace used and worn O-rings, tighten loose fittings or replace any damaged fittings.

■ Utilize 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 achieve OEM level, it is critical to ensure the right parts are used.

It’s 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, motors sometimes 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 risk of cavitation. Cavitation can cause a reduction of oil film between the metal wear parts.

Also, 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 third-party local repair shops working on a range of industrial hydraulic equipment.

■ Preventive maintenance programs. Many of the best practices described here can be incorporated into comprehensive preventive maintenance programs that identify risks and correct issues before they lead to failures or downtime. Effective maintenance programs include annual major inspections and quarterly minor inspections carried out in the field by factory-trained and certified technicians.

Good preventive maintenance programs include detailed inspections of all accessible equipment, oil temperature, pressure and non-intrusive 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 preventive maintenance program will also supply a detailed report of findings, recommended maintenance, spares and follow-up actions.

Working with experts

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 aggregate operations. Proper maintenance programs from service facilities operated by or authorized by the equipment’s OEM make good sense in the long run. These facilities have the technical personnel, 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 pit and quarry operators when it makes sense to retire legacy equipment once it becomes cost-prohibitive to continue remanufacturing. They can help develop a cost-effective direct-drive upgrade/migration plant, based on the knowledge of the unique needs of specific locations, operations and production equipment. That is one more way to leverage the maximum value.


Timothy McCrea is market segment manager of aftermarket in Bosch Rexroth Corp.’s Hägglunds’ group.


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