Design of a POWER WINCH
Other details
Abstract
Mechanical Engineering forms the backbone of all manufacturing industries. By applying the laws and norms of mechanical design, engineers validate theories formulated by scientists. Unlike the research man, the engineer can only go in for the solution till the extent the same is feasible, not only technically but commercially too.
We are now going through the second industrial revolution. In our generation, there has been a complete conversion of engineering to the digital form, thereby making theories and white papers, designs etc. accessible to everyone. The Mechanical Winch
or Hoist is a simple device of extraordinary utility. The share of Material Handling Equipment has played a tremendous role in this Industrial Revolution. When a winch is operated, it seems very elementary but take a look at what is inside ..the precision of the Planetary Gear Box, the simplicity of the NRR Ratchet and Pawl and when combined with the automation possible today through logic controllers, the winch will in no way be inferior to its odd shaped brothers.
In this paper, we have placed more importance on the possibilities of design. Instead of applying a formula and writing down the answer, we have tried to bring the detailed configurations possible on the Mechanical Winch. A summing up of the above sentiments may be best summed up by the following phrase. Do not give a man fish, but teach him how to fish. We too have adapted a similar approach of drawing a picture of the unlimited potential of mechanical design, instead of solving a problem by strict formulae.
Introduction
A Winch (also called hoist for lifting and replacing jobs) is a mechanical machine, which is capable of exerting very high pulling and/or lifting forces on objects which require repositioning in company warehouses or loaded onto a wagon or truck, at site etc. It can with ease, meet the challenge of heavy objects which need to be handled. The handling is done via hooks or D-clamps, which are connected through steel wires or reinforced non-breaking ropes, which are connected to the object by means of a hook and sling. The output force is much more than the input force as energy transfer through gears. Winches are used in material handling industries extensively but it has many other applications too.
Winches can be static type i.e. fixed to a foundation or vehicle mounted type, which are obviously portable. In ships and boats, the static winch is a common sight. In marine applications, winches are called Capstans. We also have double drum winches where with the help of a clutch, one drum can be kept separate from the other, if the load does not justify using both the drums
Fig1. A Pneumatic Winch mounted
directly on a pneumatic pressure booster
Fig 2: A Manual Winch very commonly used
while laying cross-country pipelines
Winches can be driven by various equipment ranging from manual type Engine driven, Hydraulic, Electrical Motor, Pneumatic etc. Of these Electric and Manual Winches are common.
Winches (also called hoists) for lifting have been used by man since centuries. This fact has been established by finding literary mention of them in 450 B.C. and in 4 B.C. by Aristotle, as per Ray(2007). It is believed that the great pyramids of Egypt were built with help of winches and one has to agree with that, because no human agency would have been able to lift the stone blocks, which were used to construct the pyramids of Giza.
2 REPORT
The objective of this report is to design a Power Winch, as per the given parameters. The material specifications and other parameters as required, whatever has not been specified, has been assumed as per good engineering practices.
2.1 PRINCIPLE OF OPERATION
The operating principle of a winch is simplicity itself. It basically consists of a drum, which rotates around a circular mounting shaft the c/l of which is the same as the c/l of the drum. The wire, cable, metal wire (whatever is being used) remains rolled up on the spool forming layers. The rotation is brought about by applying a torque to the central shaft. The wire rope, cable, metal chain or whatever is being used, to connect the winch to the load is done through a gear train, which converts the high speed and low toque input of the drive into the spool to a low speed high torque output to the driven shaft. Therefore, the winch is able to exert a much higher force –pulling force or lifting force on the load. Normally, manually operated winches generally tend to be either pulling type or lifting type. Electrically operated or other external drive powered winches or hydraulically powered winches can serve multiple applications i.e. both lifting and pulling. Light winches in sailing boats can be gearless too, as shown in the adjacent illustration
2.2 The Design Process
a) Listing given or known parameters
1. It is given that the winch design load capacity is 2.0-10.0 MT.
The rating of the winch in actual operation should be restricted to 5.0 MT (taking a FOS of 2) as the maximum load applicable.
2. The winch shall be electric motor driven.
No other data has been given in the specifications.
b) Calculating and/or selecting the other parameters.
(i) Mechanical Load Calculations:
Using the data given we refer to Table 2(ref Annexure 1) for finding out the dimensions, type etc. of the various sub. Assemblies, which will be required, we arrive at the following figures.
Cable diameter=0.032m
Drum diameter=1.8 MTS
Pushing /Pulling load= sufficient
Feet provided= Yes
Driver energy = AC with VFD
Active Layers =12
Used Layers=10-11
RPM ratio=2.2
In the past, these used to be calculated taking into account the hoop stress generated as well as the tensile and compressive stress on
the frame of the spool. However, we have today the advantage of charts which give us the values we want as per the force which the
drum bears. As the drum diameter is given, correspondingly all the other above parameters can be found out easily
Again , RPM ratio=2200/1000=2.2
Max RPM=2200
and Minimum RPM = 1000.
In this particular design, we show power distribution through belt and pulley. We have here considered stepper pulleys or pulleys
which increase in diameter or decrease in diameter from whichever side you look. So that the velocity of the force can be altered by
decreasing or increasing the pulley diameter. Why this is necessary? Because the presumption that the lifting of the load or lowering
of the load is just that presumption. In actual practice it is not so. If we pull the rope at a slower rate than or release at a faster rate then
at times the spool top will show an uneven wrapping of the steel rope. This will adversely affect the winch pulling system and the load
the wire rope has to be at a taut tangent to the spool at all times. The RPM change is made possible due to the belt pulley diameter of
the system. This does the job.
In modern winch designs , the motor is connected to the spool vide a flexible coupling. The velocity change can be done by a geared
Fig 4: THE SPOOL SHAFT AND THE PULLEYS AT EITHER END (Key and Keyways not shown here)
motor application or the best but also the most expensive of them all, the variable frequency device. This is an electronic device which
has probes gauging the Speed of the motor, which provides a digital feedback to the control unit. The system then changes its own
frequency by changing the poles it chooses to enable, vide the equation
N(RPM) = 2x60xʄ / P, here
P=no of poles and ʄ = frequency in Hz.
(iii)Calculating the required power
What is the energy that will be required at the spool (the shaft of the winch) end?
This power is given by the following formulae
BKW. out required=[ Line Pull (lb.) x Line Speed (ft/min)]/44236(I)
Or, BKW out required = [Output torque (in-lb.) x Output Speed (rev/min)]/84450..(II)
Winch efficiency is given by
Ƞ =(Power Output)/(Power Input) --------------------(iii)
The Efficiency depends a large extent upon the gear system of power transmission used. Almost 75% of winches use the planetary type of gearbox because of its higher efficiency. The arrangement also requires less volume as it is compact. For a perfectly selected winch, it is about 80%. We have therefore considered the same
Hence, Winch efficiency =80%
Other gear train systems used such as spur gear train, worm and worm gear arrangement etc. are less efficient and give only about 65% to 70% efficiency. In addition to which, volume requirement or space requirement is also high
Now, efficiency being 80%,
The adjacent picture shows a typical planetary gear arrangement. It can be easily guessed why it is called “planetary” from this view. The gears are arranged round a central gear, engagement and non-engagement being decided by the action of a clutch
Hence from equation 1, drive rating should be P= 25.867/0.8
Power required = 32.33 KW
2.2d) Power Transmission between shafts
We want to use a belt drive for the motor shaft, we therefore take an extra 10% efficiency loss, approximately
Motor rating required =35.92 KW
Therefore we can use motors of the following standard rating 40KW at minimum RPM of 1000. This is the worst possible condition.
Motor transmitting power by
stepped pulleys
Fig 5
There is also another electrical driving system where
the motor is integrated with the winch. Wherever VFD has to be
fitted onto the system, then the drive Motor is coupled
onto the winch shaft by flexible coupling. We are considering
stepped Pulleys so that speeds of the driven shafts can be changed.
These are the conventional ways. However, if you have ample funds and the hoist is more than a material handling device for you.
you can also go in for the geared motors as shown in the picture below (Siemens make gear motors brand name ‘MOTOREX’ are shown below at different shaft configurations)
SIEMENS make
MOTOREX geared motors. Both custom-built as well as standard motors are available
Although these are geared motors, they are still not completely smooth with RPM change. Hence, we have PLC controlled servo motors connected to reduction planetary gearing. The entire system is compact and snugged inside a Stainless Cylinder. The gears come with grease and no further lubrication is required. Picture of an Interroller make Servo motor with gears inside
As is evident, these hoists or winches will only be in very valuable applications
The world’s largest electric winch can handle 275 MT of load at a time, is shown in the adjacent picture. As can be seen, the winch is mounted in an oil exploration offshore rig in the Pacific Ocean. The winch is used for material handling/pulling pipe sections from the bore hole
3. Other Accessories Required
- NRR assembly: A winch can be very dangerous if it turns both ways around. To prevent this, a ratchet and a pin arm is used in a single contraption called the NON-REVERSE RATCHET (NRR) ASSEMBLY.
Fig.7
A small size NRR assembly disk/ratchet is shown above. If the central shaft is the spool shaft, then from the photograph it can be made out easily. The ratchet plate ensures clockwise movement only. It cannot rotate anti-clockwise.
- A Variable Frequency Drive (VFD) which changes the motor speed by changing the frequency of the input power in induction motors.
- A remote control for operating an overhead winch/ hoist in a warehouse or factory. The hoist is mounted on overhead I-beams or concrete beams.
- Automation: Control system with a PLC, sensors relay and a HMI interface. Automation and the degree to which it is provided depend upon the sophistication of the application.
For safety, a winch is usually provided with a hand brake system, and a manual over-ride all systems shut-off system for emergencies. Light Curtains can also be provided on one side of a stationary winch.
- CONCLUSION
This is the elaborate process of the design of a winch. Winch bodies are ideally AISI 316 .i.e. the material has to be extremely hard and largely immune to corrosion.CF8M or SS316 serves the purpose well. Lubrication is very important in a winch because of excessive heat buildup within short time periods. The Gear assembly is usually kept well-greased and the metal rope too too minimize friction. The ends of the spool generally are fitted with a taper roller bearing on the drive side and a heavy duty ball bearing on the NDE end before the flanges. The bearing housing are filled with lubricating oil as per manufacturer’s recommendation so that about 60% of the bearing remains inserted in it. The Emergency switch can also be solenoid operated either AC or DC. This is the design procedure of an electric Winch.
Pleases check the appendixes where we go onto the hook design, the factor of safety (FOS) concept and the hook design (although the last cannot be said to be the accessory for a winch alone).
REFERENCE:
- Markey.M (2000); Single Drum Winch Design; Electronic Copy of Chapter 10, Retrieved from
- Ray.S (2007); Introduction to Material Handling, New Age International, New Delhi
We will take Carvbon Steel C-20 as the hook material or CF8M. Mild steel is not brittle but we do not want any kind of elogantion.
Our hook will lift a load of maximum 50% of design load i.e. 5 MT.
Now from the adjacent illustration, we have C as the critical dimension.
If the value of C is known , then the others can be calculated by high school mathematics. For this, we refer to the ISO code for crane hooks
Here, from the design code we find that c =( factor of material x nearest recommended load)x √proof load
With these C is found out and the other dimensions calculated
( The ISO code is in SI units while we still follow the FPS units. Attention must be given to this while designing.)
******
ANNEXURE III
FOS (or Factor of Safety):
What is the FOS: Engineers do not design a system for artistic or aesthetic purposes but because of its utility. A machine designed for advantage has to be advantage. If it does not work, than the engineer has made a mistake somewhere. Moreover, for simplification, we usually neglect negligible values. However, all these bunched together can affect the result.
There is no calculation to determine the FOS in engineering. It depends upon the machine and the consequences of failure. This is not to say that we should take a very high FOS and relax because in that case, the design is fail proof. If we take a factor of safety 10, the cost will rise about 500%. The designed machine will not fail but it will not be economical and scrapped. It is because of this that the word ‘optimization’ is a word every engineer is fond of, since optimization has to be done every day by him or her.
