Skyline tension and dynamic loading for cable yarding comparing conventional single-hitch versus horizontal double-hitch suspension carriages

Figures & data

Figure 1. The test set-up running the single (A) and double carriage (B)

Figure 2. Examples of high-dynamic loading events. These can occur when loads get hung up on a stump or tree during inhaul or during the release of the carriage clamp under a load. Time on the x-axis refers to the duration of the respective yarding cycle, with the origin equaling the beginning of the cycle

Figure 3. Example of observed tension during a complete yarding cycle, in which high-dynamic loading occurred, with selected work tasks (outhaul, breakout and inhaul) and waypoints (midspan, support) annotated. During phases in which the carriage was not moved (between outhaul and inhaul), e.g. lowering the dropline, connecting the chokers to the load, unloading or re-positioning of chokers after the high-dynamic load, tension varied only marginally. At the end of the cycle, tension dropped back to pre-tension level

Table 1. Results of the Chi-square analysis for the frequency of events

Treatment		Support	No Supp	Midspan	No Mid	HighD	No HighD
Double	Actual #	45	34	54	25	8	71
hitch	Expected #	38	41	62	17	5	74
	Contribution	1.21	1.13	1.03	3.76	4.67	0.33
Single	Actual #	29	45	70	9	2	72
hitch	Expected #	36	38	62	17	5	69
	Contribution	1.29	1.21	1.03	3.76	4.99	0.35
Chi-Square		4.83		9.59		10.34
P-Value		0.028		0.002		0.001

Actual # = actual count of events; Expected # = expected count of events; Contribution = contribution of factor to total Chi-square value; Support = the cycle includes passing over the intermediate support; No Supp = the cycle does not include passing over the intermediate support; Midspan = the cycle includes passing through midspan; No Mid = the cycle does not include passing through midspan; HighD = the cycle includes a high-dynamic load event; No HighD = the cycle does not include any high-dynamic load events.

Table 2. Skyline tension when carriage was at midspan

Treatment		Double-hitch				Single-hitch				U-test
		n	Mean	Median	Max	n	Mean	Median	Max	P-Value
TMS	kN	54	150	150	180	70	129	128	145	<0.0001
TMS TIF	%	54	38	36	63	70	26	25	42	<0.0001
TMS/SWL	%	54	106	106	127	70	91	91	103	<0.0001
PTMS	kN	54	153	155	182	70	138	138	164	<0.0001
PTMS TIF	%	54	41	41	67	70	34	32	62	<0.0001
PTMS/SWL	%	54	108	109	129	70	97	98	116	<0.0001
MCLA midspan	kN	54	7	6	28	70	17	16	52	<0.0001
MCLA non-midspan	kN	36	12	10	25	27	19	19	30	<0.0001

TMS = tension midspan, being the average tension as the loaded carriage passes through midspan; TIF = tension increase factor, i.e. (tension minus SPT) divided by SPT*100; SWL = Safe Working Load, i.e. minimum skyline breaking strength divided by three; PTMS = peak tension midspan, being the highest tension recorded as the loaded carriage passed through midspan; MCLA = greatest peak to trough change in skyline tension during any part of the cycle, being the largest cyclic load experienced during the cycle; U-test = p-value, according to Mann–Whitney non-parametric test.

Table 3. Regression equations for predicting skyline tension when the carriage was at midspan and MCLA

Tension midspan (TMS)
TMS (kN) = a + b * SPT + c * Load + d * Double
R^2 adj = 0.827, n = 121, RMSE = 5.867
	Coeff	SE	T	P-Value
a	10.11	8.75	1.16	0.2510
b	1.06	0.08	12.8	<0.0001
c	0.01	0.01	8.95	<0.0001
d	12.79	1.20	10.6	<0.0001
Peak tension midspan (PTMS)
PTMS (kN) = a + b * SPT + c * Load + d * Double
R^2adj = 0.688, n = 150, RMSE = 7.817
	Coeff	SE	T	P-Value
a	23.89	10.75	2.22	0.0277
b	0.99	0.10	9.74	<0.0001
c	0.01	0.01	8.40	<0.0001
d	10.25	1.45	7.19	<0.0001
MCLA midspan
MCLA (kN) = a + b * Load + c * Double
R^2 adj = 0.303, n = 121, RMSE = 8.596
	Coeff	SE	T	P-Value
a	9.89	1.98	4.99	<0.0001
b	0.01	0.01	4.23	<0.0001
c	−10.45	1.59	−6.55	<0.0001
MCLA non-midspan (for cycles through midspan)
MCLA (kN) = a + b * Load + c * Double
R^2 adj = 0.470, n = 61, RMSE = 4.807
	Coeff	SE	T	P-Value
a	14.72	1.55	9.47	<0.0001
b	0.01	0.01	3.63	0.0006
c	−9.10	1.27	−7.14	<0.0001

SPT = skyline pre-tension in kN; Load = payload weight in kg; Double = indicator variable for the double carriage = 0 if single, 1 if double; RMSE = root-mean-square error (or deviation); SE = standard error; MCLA = greatest peak to trough change in skyline tension during any part of the cycle, being the largest cyclic load experienced during the cycle.

Figure 4. Point scatter and regression graph for mean tension of the skyline when the carriage is at midspan (TMS). The graphs were calculated using the equation in Table 3, for the mean pre-tension of 109 kN for the double-hitch carriage and 103 kN for the single-hitch carriage

Table 4. Pre-tension, high-dynamic load, and maximum tension during breakout

Treatment		Double-hitch				Single-hitch				U-test
		n	Mean	Median	Max	n	Mean	Median	Max	P-Value
SPT	kN	79	109	110	120	74	103	101	111	<0.0001
PTBR	kN	79	146	146	184	74	141	138	181	0.0235
PTBR TI	%	79	35	32	80	74	37	34	88	0.3899
PTBR/SWL	%	79	103	103	130	74	100	98	128	0.0235
HighD load	kN	8	156	161	168	2	169	169	184	<0.0001
MHDL	kN	8	32	32	48	2	68	68	84	<0.0001
HighD load TIF	%	8	45	43	63	2	64	64	80	<0.0001
HighD load/SWL	%	8	111	114	119	2	120	120	130	<0.0001

SPT = skyline pre-tension; PTBR = peak tension during breakout; TIF = tension increase factor, i.e. (tension minus pre-tension) divided by SPT*100; SWL = Safe Working Load, i.e. minimum skyline breaking strength divided by three; HighD load = sudden and extreme tension peak followed by a tension drop; MHDL = magnitude of the high-dynamic load, being the difference between the high-dynamic load tension and tension just prior to the event; U-test = p-value, according to Mann–Whitney non-parametric test.