Evolution of an Erector Model: The Stiff Leg Derrick

Hi folks, Doc here. Welcome to the third installment of "The Evolution of an Erector Model". This time, the subject is the Stiff Leg Derrick. Lifting models of various types, including cranes, derricks, and other hoisting devices, featured prominently in Erector instruction manuals through the years (I will profile all of these models in another feature). One particular such device, the stiff leg (or "stiffleg") derrick, appeared repeatedly over time, changing very little with each new iteration. The first appearance of this derrick was at the very beginning of Erector history in 1913. It appeared again during the Classic Period in its most impressive form, due in large part to the special parts that were developed just for the model. This version was quickly followed by a simpler version, and then in later years by smaller and still simpler versions. In each case, however, the model design retained the structure and features that define the stiff leg derrick.

WHAT IS A DERRICK?
Derricks, like cranes (to which they are similar in most respects), are industrial machines that are generally used to lift or lower materials and to move them horizontally. Their primary function is lifting heavy objects and transporting them to other places. However, unlike cranes which are generally (but not always) mobile, derricks typically feature stationary foundations or bases. A guy derrick (also known as a boom derrick) is a fixed mast derrick that can be rotated and connected to a lifting boom. The mast is typically in a vertical orientation with a base that allows the mast and attached boom to rotate. Multiple guy wires attached to the top of the mast are anchored to the ground to support the load. The bottom end of the lifting boom is usually connected to the base of the mast, from which it extends outward and upward. A pulley/cable system connecting the top of the boom to the top of the mast allows the boom to be raised and lowered. The load is lifted by a second pulley/cable system also connected to the upper end of the boom.

A stiff leg (stiffleg) derrick, also known as a Scotch derrick, is one with a boom similar to a guy derrick, but instead of using guy wires to secure the top of the mast it uses two or more stiff members, called "stifflegs" (see the photo above right). These stifflegs are capable of resisting both tensile and compressive forces. Horizontal sills like those shown in the photo are often used to connect the bottom ends of the stifflegs to the base of the mast. To facilitate heavy load lifting, counterweights can be placed on the ends of the sills behind the stifflegs.

TYPE I STIFF LEG DERRICK
This first version of the Erector stiff leg derrick (hereafter referred to as SLD), built with Type I parts, features the main structural components of a stiff leg derrick, including the boom (B), mast (M), and stifflegs (S) - see image below. This large model does not incorporate sills (highlighted in blue, as described in the previous paragraph). Instead, the mast and stifflegs are anchored to a wooden base, providing the stability the model requires. The mast and boom assembly swivels on a rotating base unit. This rotation, along with raising and lowering both the boom itself and the load being lifted, is accomplished using a hand powered control mechanism incorporationg gear, cranks, wheels, and string.

THE CLASSIC PERIOD SLD AND THE TRUMODEL CONNECTION
As Erector fans will know, and as I've documented repeatedly on this website, the Classic Period of Gilbert Erector (1924-1932) produced some of the greatest sets of all as well as a variety of very special model designs that are known collectively today as the "Classic Period Models." A variety of special parts were developed specifically for these particular models, and each one was built using a combination of these special parts and standard Erector parts. One of these models was the SLD shown in the image below. This model had an interesting origin: it began life as part of the Trumodel toy line by the Noble Manufacturing Company. Trumodel toys were replicas of real construction equipment that could be built using a variety of specialized parts. A.C. Gilbert acquired the Trumodel toy line and merged it with Erector. He then began producing his own versions of many of the Trumodel components. The Classic Period SLD (shown below along with the Trumodel version on which it is based) utilized a number of these special parts, including the AY bull ring, the BP tip bucket, and most notably the ED three-drum hoist (also shown in a separate image below). Other writers have noted that this version of the Erector SLD may be the least well known of the Classic Period Models. This seems reasonable given the fact that the model appeared in the form shown below only in 1927 because the ED hoist was discontinued after just a single year. A new modified version of the SLD replaced the Classic 1927 model the next year, and that model remained in Erector manuals for the remainder of the Classic Period. In subsequent years, the model was revised several more times; those versions are described later on this page.

LATER PERIOD SLD DESIGNS
As I noted earlier, SLD models remained in Erector manuals in one form or another for the remaining years of Gilbert Erector, two of which are shown below. The first image shows a large model that is structurally similar in most respects to its Type I and Classic Period predecessors. The primary difference is that the model has a solid base composed primarily of MN base plates to which the derrick structure is anchored (no sills are used as a result). This version is powered by an A49 motor and gearbox. It also incorporates a battery powered MJ electromagnet as the main lifting device instead of a hook or tip bucket. The second image shows a smaller (shorter mast and stifflegs) and simpler (hand powered) SLD design. This one has a base that is made primarily from two angled MN base plates that in effect function as large flat sills. The third image is from the same period, but depicts a standard boom (guy) derrick, as described in the WHAT IS A DERRICK? section above, rather than a SLD like the others shown on this page. In this case, the structure is basically the same but a boom derrick uses guy wires to support the mast rather than stifflegs.

ADDENDUM: OTHER SYSTEM SLD MODELS
Gilbert Erector was not the only system that featured SLD models in their instruction manuals over a period of many years. Meccano and American Model Builder (AMB) likewise took a particular interest in this industrial machine, and it appeared throughout the same period in their literature. The images below show two iterations of the Meccano SLD, an early version from 1916-21 and a later one - Super Model No. 6 - from 1928-37. These two versions were very similar structurally, as you might expect, but exhibit marked differences in their respective power/gearbox components and mast base and swivel assemblies. The third image shows the AMB version, which is differs structurally from the other SLDs shown on this page, although all major components are present. In particular, note that the lifting motor is part of the pivoting mast assembly.