Technology

News

U.S. Department of Health and Human Services

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

No. 480 January 2000

Controlled Start for Drill Motors on Roof Bolting Machines

Objective

To provide a practical method to reduce hand and arm injuries

during roof bolting operations.

Background

Operators of roof bolting machines have one of the most

dangerous jobs in underground mining (figure 1). The main

hazards include unsupported roof and movement of equipment and

machine appendages in confined surroundings. Many bolter

operators have been injured because their hands or gloves have

been caught by rotating drill steels or bolt wrenches. A bolter

operator must manually place the drill steel and wrench into the drill

chuck and then hold onto it as the drill is guided to the hole

placement or the wrench is raised to the bolt head. During these

actions, the inadvertent actuation of the drill's rotational control

can catch the operator's hand or glove, which can cause serious

injury.

Approach

The National Institute for Occupational Safety and Health

(NIOSH), Pittsburgh Research Laboratory, has developed a

mechanical/hydraulic method to reduce this type of injury.

Although an experienced bolter operator can sometimes "feather"

the hydraulic controls on modern bolting machines to achieve a

relatively slow rotational speed, the control designs are not

normally optimized for this. Inadvertent control actuation

invariably results in full-on operation. The main reason for these

types of incidents is that the sudden full-speed rotation of the drill

catches the operator by surprise. The technique that we developed

intentionally limits the rotational speed and torque of the hydraulic

drill motor(s) for several seconds during startup. During this time,

the operator has the chance to see that drill rotation has started and

can stop the rotation or release his/her hand before injury occurs.

The technology is suited for incorporation into new machine

designs by the manufacturer or implementation by mines or rebuild

shops on a retrofit basis.

How It Works

A controlled start of the drill motor can be achieved in many

ways. This Technology News describes three designs and their

respective advantages and disadvantages. A feature common to

all of the designs is that they use a rising pressure in an

accumulator to control the starting characteristics of the drill motor.

All of the designs use the bolting machine's existing hydraulic

pumps and drill motors.

The first design is shown in figure 2. As pressure in the

accumulator rises, the circuit provides the drill motor a reduced

flow through the flow control and limited pressure through the

pressure-reducing valve. The reduced pressure limits motor torque,

and the reduced flow causes lowered motor rpm. As the

accumulator fills, pressure builds behind the counterbalance valve.

When sufficient pressure is built up, the counterbalance valve

opens, allowing almost full flow and pressure to the hydraulic

motor. A small restriction ahead of the counterbalance valve is

needed to keep the accumulator charged, providing pressure to

keep the counterbalance valve open. The check valve in the

counterbalance valve provides for reverse motor rotation. The

accumulator is drained when the operator releases the control lever

to its center position, ensuring a controlled start each time. The

action of this controlled start is like a shift in an automatic

transmission. First there is low torque and rpm at the drill motor,

which shifts to high rpm and torque when the counterbalance valve

opens. This design is fairly simple, but not ideal because additional oil heating occurs. If this circuit were to be implemented,

one needs to carefully consider the ability of the oil cooler on the
equipment to remove the added heat. This design also precludes

the controlled start feature in the reverse rotation direction, and

maximum drill speed is slightly reduced.

Figure 3 shows another design. In this circuit, the drill motor's

initial rotational speed is very slow, then ramps up to its maximum

rpm. The size of the accumulator and the setting of the flow control

valve controls the slope of the ramp. A pilot signal is delivered to

the pilot-operated flow control from the rising pressure in the

accumulator. This causes flow to the motor to slowly increase.

Depending on the exact design requirements of flow and pressure,

the pilot-operated flow control may be difficult to procure, as these

are normally available in limited sizes. It would be possible to

substitute a pilot-operated poppet valve with a custom-machined

internal spool. A poppet valve is normally designed as an on/off

device, and its internal spool has sharp edges on its lands to

perform this function. Since flow control is desired, it would be

necessary to have the internal spool designed and machined with

tapers instead of sharp edges. This would provide the desired flow

control action and should be available for a modest cost. In this

design, as in the first, there is no provision for a controlled start

during reverse rotation. Also, the drill torque is not as closely

controlled as in the first design.

The final design, shown in figure 4, is slightly more complex, but

offers more efficiency and control. This design requires a separate

charging or pilot circuit for operation. This should not be a

problem, because most roof bolting machines have many hydraulic

circuits available. In this design, the operator controls a small flow

through a manual valve into an accumulator. Figure 4 shows an

open-center spool on the manual valve. Depending on the

application, this may need to be changed to a motor-style spool.

The pressure signal from the accumulator is connected to a pilotoperated proportional four-way valve that controls the drill motor.

This type of valve delivers and regulates flow in proportion to the

pressure on its pilot ports. The operator would have little or no

control over the starting characteristics of the drill. This would be

entirely controlled by the size of the accumulator and the setting of

the flow controls. As in the second design, the motor start is a

controlled ramp from zero to maximum rpm. An advantage of this

design is that the volume of the accumulators is much smaller than

that required by the other designs. This design provides for a

reverse controlled start with an independently controlled ramp

speed.

Patent Status

The controlled start hydraulic systems have been assembled

and tested in the laboratory to ensure that they function as

designed. Also, we have presented the concept and designs to a

major roof bolter manufacturer, who has agreed on their principle

of operation, practicality, and usefulness. This manufacturer is in

the process of implementing an electronic version of a controlled

start method, as this fits well their new electronic control system.

The mechanical/hydraulic methods described should be applicable

to traditional, hydraulically controlled bolting machines.

For More Information

For more information, contact Joseph P. DuCarme, NIOSH

Pittsburgh Research Laboratory, Cochrans Mill Rd., P.O. Box 18070,

Pittsburgh, PA 15236-0070, phone: (412) 386-6728, fax: (412) 386-

5108, e-mail: jld6@cdc.gov

To receive additional information about

occupational safety and health problems, call 1-

800-35-NIOSH (1-800-356-4674), or visit the

NIOSH Web site at www.cdc.gov/niosh

Mention of any company name or product does not

constitute endorsement by the National Institute

for Occupational Safety and Health.
Figure 1.—Bolter operator beginning drilling procedure.

Figure 2.—Controlled start design using counterbalance valve.
Figure 3.—Controlled start design using pilot-operated flow control.

Figure 4.—Controlled start design using pilot-operated proportional valve.