Mine Development & Surveying

Rock–Tool Interaction & Mechanical Cutting

Specific energy — the single number that links cutter forces, machine power and production rate for roadheaders and continuous miners.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Mechanical excavators (roadheaders, continuous miners, TBMs) break rock with drag or disc picks. The governing quantity is specific energy (SE) — the energy needed to excavate unit volume of rock. Efficient cutting (deeper picks, optimum spacing-to-depth ratio) minimises SE; blunt tools or shallow scraping raise it. Because

S E = power / production rate

, it directly ties the installed cutting power of a machine to the volume rate it can achieve in a given rock, and the cutting force on each pick to the depth of cut. SE rises with rock strength, so it is the practical index for machine selection and cutter-head sizing.

The Core Formula Matrix

Specific energy:

S E = \frac{P _{c u t}}{Q}

(cutting power

P_{c u t}

÷ volume production rate

Q

)

Production rate:

Q = \frac{P _{c u t}}{S E}

, with

S E

MJ/m^{3}

and

P

in MW gives

Q

m^{3} / s

.

Cutting work per pick pass:

W = F_{c} \times ℓ

(mean cutting force

F_{c}

× cut length

ℓ

)

Unit note:

1 MJ/m^{3} = 1 MPa

; SE and rock UCS share dimensions.

The ‘IIT Traps’

⚠
Lower SE = better cutting. SE measures inefficiency of energy use, so minimising it is the design goal.
⚠
Keep units consistent. $S E [MJ/m^{3}]$ with power in MW gives $Q$ in $m^{3} / s$ ; convert to m³/h by ×3600.
⚠
Cutting power, not total motor power. Auxiliary, gathering and tramming loads are excluded from $P_{c u t}$ .
⚠
SE rises with optimum s/d ratio departures. Too-wide pick spacing leaves ridges and spikes SE.

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

Specific energy in mechanical rock cutting is defined as the energy required to:

Q2MEDIUM2 Marks · NAT

A roadheader cutting head develops

300 kW

of cutting power in rock of specific energy

20 MJ/m^{3}

. The instantaneous cutting rate is ______

m^{3} / h

. (Round off to one decimal place.)

m³/h

Q3HARD2 Marks · NAT

A pick exerts a mean cutting force of

4.0 kN

over a

0.6 m

cut, removing

0.0015 m^{3}

of rock per pass. The specific energy is ______

MJ/m^{3}

. (Round off to two decimal places.)

MJ/m³