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Gearbox cradle
Optimisation for cost and weight

The design brief

A cradle had been built for the gearbox of a marine craft. The drive to the gearbox is from a cardon shaft via pulleys and a toothed belt. The pulley bearings are mounted on the cradle.

The cradle itself straddles a well between structural parts of the hull and transmits the thrust from the propeller into the hull.

The prototype cradle was an aluminium fabrication. Aluminium was a natural choice to save weight, which was critical, but the cradle had been expensive to make, because of the fine tolerances required between the bearing mounts.

The brief was to develop a new design which would have a similar stiffness to the original, be significantly cheaper and, if possible, be even lighter. It also needed to be easy to fit and robust in a marine environment.

Initial findings

Solidworks 3d CAD was used to model the prototype gearbox cradle. Cosmos finite element software was used to find the stress and deflections under load.

The deflection of 1.1mm due to thrust from the propeller was much more than expected. A very high stress
was predicted, concentrated in a corner. This was much higher than the yield stress, which suggested local

Key points Light weight aluminium cradle was too expensive FEA showed long-term life problems The new design used a more efficient load path and stainless steel Optimisation saved at least half the cost and 16% of the weight			Gearbox cradle Optimisation for cost and weight The design brief A cradle had been built for the gearbox of a marine craft. The drive to the gearbox is from a cardon shaft via pulleys and a toothed belt. The pulley bearings are mounted on the cradle. The cradle itself straddles a well between structural parts of the hull and transmits the thrust from the propeller into the hull. The prototype cradle was an aluminium fabrication. Aluminium was a natural choice to save weight, which was critical, but the cradle had been expensive to make, because of the fine tolerances required between the bearing mounts. The brief was to develop a new design which would have a similar stiffness to the original, be significantly cheaper and, if possible, be even lighter. It also needed to be easy to fit and robust in a marine environment. Initial findings Solidworks 3d CAD was used to model the prototype gearbox cradle. Cosmos finite element software was used to find the stress and deflections under load. The deflection of 1.1mm due to thrust from the propeller was much more than expected. A very high stress was predicted, concentrated in a corner. This was much higher than the yield stress, which suggested local
			yield and stress relief would occur. There was a high risk of work hardening and fatigue or stress corrosion cracking over time. No problems had been experienced with the prototype cradle, but it had run for very few hours. New concept A new design was worked up, based on laser cut and folded steel plate. The design would be much cheaper to make and, with no welding, the necessary tolerances would be easier to maintain.
			Stainless steel is advantageous in a marine environment, because it is much less prone to corrosion and work hardening. Making it lighter than the aluminium version would rely on optimising the position and thickness of the members and cutting out surplus material. Straddling the structural members of the hull was necessary, but transferring the thrust from the gearbox to the top of the cradle and then sideways to the hull was inefficient. A new layout was employed, which transferred the thrust sideways in a single member, allowing lighter members to carry the bearing loads.
			Detail design Cosmos was used to model the loads in the new folded members. This highlighted a problem at the sides of the folded thrust member, where the loads were transferred to the hull. In order to optimise the design of the thrust member, the detail of the mounting bracket had to be changed. The final design is a bolted assembly of folded laser cut plates, in Stainless 316L. The manufacturing cost was reduced by at least a factor of 2 and there was also a 16% weight saving.

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