Selection (& Design) of Equipment for Towing

Background

• Recent serious towing related accidents at mines:

– Safety Alert 156 Towing of longwall chocks almost results in fatalities, 13th February 2007, Queensland department of Mines & Energy.

– MSHA report CAI-2003-24, Fatal powered haulage accident, September 9, 2003, MSHA

– Safety Alert 217 Unplanned movement of towed equipment, 9th January 2009, Queensland department of Mines & Energy.

– Safety Bulletin SB09-03 Broken pull chain results in fatality, 23 September 2009

Background continued…

• Department of Industry and Investment releasing Safety Bulletin SB09-03 recommending: – all mines review their towing, pulling and snigging operations; and – consider “potential impact forces on the towing system resulting from

dynamic loading”.

• Lack of information regarding the dynamics of towing in mining equipment.

• Proposed changes to MDG-15: – Remove “minimum Factor of Safety = 2.5” – Add requirement for a risk assessment to determine the towing

method – Compulsory use of towing ‘strops’

Hazards • Two primary hazards:

– Unplanned movements – Sudden release of stored energy

• Recoil of broken connectors is very dangerous

• It may help to consider a towing connector as a big rubber band

• The stored energy in towing connectors can be massive – Depends on several things – Typically, more energy than a rifle being fired

Towing Strops • ‘Strops’ – towing connectors which ‘fail safely’.

– Do not recoil violently when broken – Typically has a highly-ductile energy-absorbing component – Specially developed synthetic fibres

• Only a partial solution – May still result in unplanned movement – ONLY effective if the strop is the WEAKEST LINK!! – Using a strop is not an excuse to become ‘complacent’

Real News Headlines • Broken tow rope 'like slingshot‘, killing boy in

back seat. Metal hook was the projectile Brisbane Times, December 14 2010, Hervey Bay.

• Woman killed in freak towing accident Tow Ball was the projectile ABC News, 21 Aug 2011, Geraldton.

• Flying tow strap hits teen on head after 4WD bogged in Castlereagh Shackle was the projectile Sydney Morning Herald, July 13 2015, Castlereagh.

Comparison, Lifting and Towing

LIFTING TOWING

LOAD

Current Requirements Lifting Towing

Standards AS1418 and others Only MDG-1, MDG-15

Static load (nominal load) Mass of object Doesn’t exist

Peak load (dynamic load) (1.5-2.0) x mass of object Maximum tractive effort

Load cycle frequency Generally, one per lift Undefined, but more

Minimum fatigue life 20,000 cycles No requirement

Minimum “Factor of Safety” Works out to be 2.5 for rigid parts approximately 4.0 4.0 for safety chains

“Factor of Safety” Approach to Lifting

Factor of Safety = 4.0

Dynamics (1.5 to 2.0)

Load (Tonne) Force (kN)

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Stress (MPa)

Static Dynamic Fatigue Yield Material load load strength strength strength

(200 MPa) (300 - 400 MPa) (384 - 500 MPa) (640 MPa) (800 MPa)

REAL Long-term F.O.S. 1.0 to 1.7

…against fatigue failure (20,000 cycles)

REAL Short-term F.O.S. 2.0 to 2.7 …against one-time overload

(Strength failure)

“Factor of Safety” Approach to Towing

Factor of Safety = 2.5

Dynamic Load ??

Load (Tonne) Force (kN)

0

`

1.5 3.0 0.5 1.0 2.0 2.5 4.0 Stress (MPa)

Maximum Fatigue Yield Material Tractive strength strength strength

(384 - 500 MPa) (640 MPa) (800 MPa) Effort (320 MPa) Dynamic

load (???? MPa)

What does a dynamic factor of 2.0 look like??

18.3T 17.0T

75mm

16.0T Impact Force

Research Project

• ACARP funded research project:

– “Tow Force Measurement of Various Mining Equipment”

• Part 1 (2014/15) – Surface testing (Currently available on ACARP website)

• Part 2 (2016/17) – In-service testing (Active testing at Centennial Springvale)

– Aim:

• To measure tow forces and publish results

• Simplify the results into guidelines (rules/charts/tables)

Instrumented Tow Pin (Stage 2)

Part 1 Results

Part 1 Results continued…

Test No. Te st 1 Te st 2 Te st 3 Te st 4 Te st 5 Te st 6

Towing machine

Tractive capacity (kgf)

Towed item Trailer

Towed mass (kg) 7,500

Nominal tow force (kgf) -

Friction coefficient on dirt -

Connection Chains Chains &

strops Chains Chains &

strops Rigid Rigid

Peak measured tow force (kgf) 16,348 14,554 11,651 14,110 n/a 9,273

Peak force / nominal force 2.32 2.06 - - n/a -

Peak force / tractive capacity 0.80 0.71 0.57 0.69 n/a 0.45

Peak force / towed mass 0.96 0.86 0.81 0.98 n/a 1.24

Peak force / tow vehicle mass 0.89 0.80 0.64 0.77 n/a 0.51

Peak force / ISO18868 D-Value 0.95 0.90 0.82 0.89 n/a 0.75

8.55 7.51 6.01 6.55 n/a 12.63 Fatigue Damage Rate (Full-load cycles per hour)

17,000 14,400

Coal tram CT-10

20,500 kgf

Sled Eimco 913

7,052 -

0.41 -

Part 1 Results continued…

Peak Towing Force

Peak

Tow

ing

Forc

e (k

g-fo

rce)

24,000

20,000

16,000

12,000

8,000

4,000

0

Mass Ratio x Dynamic factor

ISO 18868 Eq'n

Maximum Tractive Effort

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000

Mass of Towed Item (kg)

Revised Approach to Towing

Factor of Safety = 3.0 Peak Dynamic

load (270 MPa)

Stress (MPa) Load (Tonne)

0 100 200 300 400 500 600 700 800 Force (kN)

Dynamic Max. Tractive Fatigue Yield Material load Effort Strength strength strength

(40% - 100%) (270 MPa) (100k cycles) (640 MPa) (800 MPa) (270 - 500 MPa)

Part 1 Results continued…

Existing Expected Recommendation

Reference load (RL) Max tractive effort (MTE) MTE or Tractor mass (m1), whichever is greater

Min “Factor of Safety” Hitch: 2.5 x MTE Chains: Rated for MTE

Hitch: 3.0 x RL Chains: Rated for 1.5 x RL

Minimum fatigue life

Fatigue load frequency

Fatigue load magnitude

No requirement

Not defined

Not defined

- 100,000 cycles for machines intended to tow regularly

- Nil for recovery

10+ cycles per hour

k determined from test results: k (m1 X m2) (m1 + m2)

Testing is currently ongoing – all details to be confirmed. Results will be published in ACARP Project #C25071 report.

Acknowledge Wrong to make Right:

• Towing equipment failures are still occurring – and theyare very dangerous.

• “Factor of safety” term is often not well understood andcan be misleading

• A factor of safety (strength-based) approach may not be sufficient for towing equipment designed to be usedregularly – Fatigue life requirements should be stipulated.

• Lack of information regarding towing dynamics – whichwe are working to remedy.