NZR G class (1928)

NZR G class (1928)
An NZR G class locomotive in Garratt form
Power type	Steam
Builder	Beyer, Peacock & Co., NZGR, Hillside Workshops
Serial number	BP: 6484–6486
Build date	1928
Configuration	Garratt: 4-6-2+2-6-4 Pacific: 4-6-2
Length	69 ft 8 in (21.23 m)
Width	8 ft 6 in (2.59 m)
Height	11 ft 6 in (3.51 m)
Locomotive weight	136 long tons (138.2 t)
Boiler pressure	Garratt: 200 psi (1.4 MPa) Pacific: 180 psi (1.2 MPa)
Cylinders	Garratt: 6 Pacific: 3
Cylinder size	24 × 16.5 in (610 × 420 mm)
Tractive effort	Garratt: 51,580 lbf (229.44 kN)
Career	NZGR
Number in class	Garratt: 3 Pacific: 6
Number	Garratt: 98 - 100 Pacific: 95 - 100
First run	Garratt: 1928 Pacific: 1937
Last run	1956
Preserved	0
Disposition	Withdrawn

The NZR G class was a type of Garratt steam locomotive used in New Zealand, the only such Garratt type steam locomotives ever used by New Zealand Government Railways. They were ordered to deal with traffic growth over the heavy gradients of the North Island Main Trunk and to do away with the use of banking engines on steep grades. They were one of the few Garratt designs to employ six cylinders. A mechanical stoker was used to feed coal into the locomotive.

Introduction

In 1928, New Zealand Government Railways obtained and operated three unusual Garratt locomotives in the 4-6-2 + 2-6-4 layout from Beyer, Peacock and Company of the United Kingdom. These engines had three cylinders (24 inches x 16.5 inches) on each of the two set of engine frames, thus creating a 6 cylinder Garratt. The engines entered service in 1929.

Walschaerts valve gear operated the outside cylinders with the inner third cylinder operated by a Gresley mechanism. The locomotives proved a disaster on the light NZR tracks. W. W. Stewart, in his book When Steam was King (pp. 98-104) suggested the most likely reason was because the engines were too powerful for the system and also the valve gear mechanisms were complicated. Stewart stated, and existing photos verify, that the design was most unusual in that the coal bunker was carried on an extension to the boiler frame rather than the normal Garratt positioning on the rear engine frame.

The engines operating at 200 psi and delivered 51,580 lbs of tractive effort, which on the lightly laid New Zealand tracks, proved to be too powerful for the drawbars on rolling stock, and broken drawbars occurred wherever the engines ran. Further, the locomotives when hauling a full load, generated such intense heat in restricted tunnels, which are common in New Zealand, that crews refused to work them.

Withdrawal

Consequently, the load was reduced and so defeated the purpose for which the Garratts were purchased; namely to operate heavy loads over a vital main line section of the NIMT route, the central section including the Raurimu Spiral. The trailing engine axle under the cab carried a heavier load than the leading engine trailing axle and experienced continual problems with overheating. Also the coal bunker carried insufficient fuel in-service and this problem was never remedied, because it would have increased the axle loads beyond the light trackage capabilities.

In 1931 the engines were withdrawn from service. Their numerous design faults sealed these locomotives' fate when the K class was introduced in 1932.

Rebuilds

In 1937 the three ill-fated Garratts were dismantled at Hutt Workshops, and some parts were incorporated into six G class three-cylinder Pacific locomotives constructed at Hillside Workshops. The engines were provided with an A^B class boiler, a new roller bearing trailing truck was fitted, also a new cab, and a Vanderbilt tender of similar design to that of the A^B class - although of larger and all-welded construction. All bogies on the engine and tender were equipped with roller bearings, and other modern equipment included power reversing gear and exhaust-steam injectors. The three cylinders retained their conjugated valve gear from the Garratts, but this gear was rather light in construction and gave persistent trouble in maintaining the correct settings. The locomotives were used in the South Island mainly on the Christchurch to Arthur's Pass section of the Midland line and mainly in freight service. The class was far from popular and their shortcomings far too numerous to justify the high cost of major improvements.

External links

• Photo of a Garratt locomotive steaming out of Wellington on preliminary trials
• NZR G class - Garratt
• NZR G class - 4-6-2