Garrett articulation

[Derek Walker]

A stupid question, the GMAM Garrett articulates at the front, but does it have an articulated joint at the back too? what sort of radius bend would a Garrett be able to take compared to a conventional non-articulated loco? And, if there is a joint at the coal bunker side, how does that affect the automatic stoker? does its mechanism articulate as well?

[Aidan McCarthy]

Hi Derek,

Yes a Garratt locomotive articulates at the front and the back. A Mallet locomotive only articulates at the front. The mechanical stoker has a ball joint where the elevator part connects to the trough. This is required for non-garratt locomotives as the stoker is in the tender and the tender is not rigidly fixed to the locomotive.

Cheers

Aidan

[Steve Appleton]

Derek, you have to think of a Garratt locomotive as though it comprises two ordinary locomotive engines with a boiler pivotted and suspended between them. Each of the two "engines" is a rigid equivalent to a similar ordinary locomotive, excluding the tender. This means that the tightest curve that can be negotiated by a Garratt is determined by the rigid wheelbase length of each of the front and rear engines. In general, the bigger the Garratt loco, the longer each of its engines' fixed wheelbase will be. Therefore, the larger the minimum radius curve it can traverse will increase too. A 2-6-2 + 2-6-2 GD Garratt would, in general terms, be able to negotiate a smaller radius curve than a 4-8-2 + 2-8-4 GMAM Garratt, which in turn would be similar to that which a 4-8-2 normal loco could negotiate. Of course the Garratt is symetrical and can thus run equally well forwards as backwards, eliminating the need to turn at each end of a trip.

So, the Garratt articulation does not directly solve the rigid wheelbase length problem. What it does do though is increase the tractive effort available through having two engine units whilst, at the same time, keeping the axle-load down by sharing the weight of a single boiler between them. Thus, a Garratt requires only one crew and could in theory run on lighter track than a pair of equivalent normal locos. In practice, the lighter track advantage was lost in the GMAM class because their massive size and heavy axle loading requires heavy "mainline" track. Only the greater tractive effort and single crew advantages remain.