Twin input/ Single output gearbox calculation help 1

[ameenbassam]

I want to do some calculations for two diesel engines drive a ship propeller through a gearbox. I want to know the reduction ratio, total inertia of the system, torque ratio, etc.

I know that the power delivered at the propeller which rotates at 151 rpm will be equal to the sum of power of the two engines running at 720 rpm.
P(load) = 2*p(eng)
T(laod) * 151 = 2*720*T(eng)

is (2*720)/151 the correct reduction ratio ? or it is just 720/151 ??
and how can i calculate total inertia of the system ??

Many Thanks in advance

 

[drawoh]

ameenbassam,

From engines to propeller, your drive ratio, obviously, is 151/720=0.210. The torque ratio, ignoring efficiency, is the inverse of that, 720/151=4.77.

Total inertia of what system? You have provided us no information whatsoever.

What are you trying to do? Your questions are something like first day of the first semester of a machine design course, if you took any sort of mechanical engineering. Okay, maybe the second day. If you are specifying expensive hardware and/or there are safety issues, there is a lot more stuff you need to know.



--
JHG

 

[MikeHalloran]

Normally, boats use two engines for reliability, and to that end, make the engines as independent of each other as possible, right down to multiple gearboxes, multiple propellers, multiple electrical and control systems, multiple day tanks, and multiple seawater systems.

Your proposed gearbox design, while certainly possible and also very much nontrivial, reduces the reliability of the propulsion system by introducing several single points of failure (everything from the gearbox to the propeller), and also introducing the expense of a custom gearbox. You would save money and be no worse off with a single larger engine driving your single propeller through a standard gearbox.

Further, if you are using non-custom parts, a few phone calls will get you the information you need to calculate total system inertia. For custom parts, you have to design them, then calculate the inertia of everything moving inside, and combine those numbers appropriately, as outlined in your machine design books.



Mike Halloran
Pembroke Pines, FL, USA

 

[ameenbassam]

Hi Drawoh
Thank you for your reply. what i am trying to do is mathematically model the shaft dynamics of this system which consists of two diesel engines driving a propeller through a gearbox. This equation should be used:
2*pi*Inertia of rotating parts*(dn/dt)= T(eng) - T(propeller) - T(friction)

Total power delivered to the propeller = 2* power engine. So, T(prop)*rpm(prop) = 2*T(eng)*rpm(eng). If the drive ratio (N) is 720:151, i think torque ratio should be (2*720):151

Regarding the inertia, i don't need numbers. I need help with the equation because the gearbox will effect the system inertia with a value of the square of drive ratio (N^2). So the total system inertia will be = inertia of propeller including entrained water + shaft + gearbox + (engine inertia*N^2) ????

 

[ameenbassam]

Hi MikeHalloran

Thank you for your reply. This system is already existing and i am just trying to model and study it.

 

[MikeHalloran]

Your model probably should include the gearbox input and output shafts and gears, also any intermediate shafts and gears, with inertias adjusted by the various ratios, etc. If you are lucky, you may be able to get that from the gearbox mfgr, else you can maybe estimate from drawings and photos. Don't forget bearings, couplings, and clutches.

Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

Using two large low-speed diesel engines coupled thru a reduction gearbox to drive a single propeller presents a very nasty design case. The instantaneous torques passing through the gearbox will vary greatly during a single revolution of each engine crankshaft. Depending on the number of cylinders and the firing order/frequency of each engine, and the crank phasing provided by the gearbox, there might be a cyclic load reversal between the two engine inputs several times per rev. This would be very hard on the gear teeth.

You need to pay attention to the instantaneous torque levels produced by the engines in addition to the inertias of the gearbox parts, shafts and propellor.

 

[ameenbassam]

Thank you all for the advices but this is an existing system and i just want to mathematically model it.
What is the gear ratio for twin input/single output gearbox ??? is the gear ratio will be different from torque ratio ??? the total system inertia at the shaft should be = inertia of propeller including entrained water + shaft + gearbox + engine inertia*(gear ratio^2) ????

 

[zeusfaber]

As a "quick and dirty", I would ignore the fact that there are two inputs to the gearbox.

The ratio isn't affected: Neither propeller speed nor the speed of either engine is changed by the fact that there is a second engine in the system.

The torque seen by the propeller shaft - and by each of the engines is the same, regardless of whether each engine goes into the gearbox through its own input gear, or the two are somehow connected to a common input gear through some sort of double-ended shaft.

For modelling, you might try to model each stage of reduction, rather than each individual train so, where you have two input gears, model them as a single input gear running at the same speed as each of the gears it represents, with mass/moment of inertia equal to the sum of all the components that are actually rotating at that speed. Similarly, model the two engines as a single engine running at the same speed, but with double the moving component mass/moment of inertia.

A.

 

[3DDave]

Model them for what purpose and with what software?

 

[ameenbassam]

Hi 3DDave
I will use the mathematical equations into Matlab. So i need to know the speed ratio and torque ratio for this system which consists of two engines running one propeller through a twin input/single output gearbox.