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April 2010 Willies Workbench: Winches

Posted in Features on April 1, 2010 Comment (0)
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April 2010 Willies Workbench: Winches

Willie's Workbench
Winches And Such
When I first started fourwheeling back in the dark ages (it was referred to as "jeeping" then), very few people had winches mounted on their vehicles. There were a few PTO winches available from long-forgotten names and some electric winch companies, but the ones I had any contact with were usually homemade modified military-surplus "bomb winches" with 12-volt starter motors attached in place of a crank.

Just in case you don't have a clue to what a "PTO" winch is, perhaps an explanation is necessary. PTO (power take-off) winches probably originated with military vehicles. A separate gearbox is attached to either the transfer case or the transmission, and from it a driveshaft was snaked past the front (or in some instances the rear) axle to a gearbox on the winch. To engage the winch, a lever inside the cab was shifted. Winch speed was dependent on engine speed. On today's 4x4s, snaking a driveshaft forward past the suspension, exhaust system and front axle is pretty difficult, and about the only place you see PTOs is on military trucks, construction equipment and tow trucks.

Arthur Warn most likely was responsible for the development and expansive use of the modern electric winch that is so prolifically mounted on the front of vehicles today. In 1959, he teamed up with the Belleview Manufacturing Company to produce the now long line of electric winches under the Warn name.

Enough of the history lesson. What I really want to talk about is winch capacity. A commonly question asked is, "How much winching capacity do I need?" Generally speaking, the minimum capacity you want can be figured as: 1.5 x (vehicle weight or pulled load)

For instance, if your Jeep weighs in at 4,000 pounds, then a 6,000-pound winch is the minimum capacity of pulling power you need. If your truck is tipping the scales at 6,000 pounds, then a 9,000-pound winch is necessary. Again, generally speaking, bigger is better (up to a point, anyway), so perhaps in some instances you would need to go to two times the vehicle weight.

Most winch pulls are well under the maximum spooled-on cable length of 75 to 125 feet. It takes at least three full layer wraps of the cable around the drum to hold this much cable. However, as the cable overwraps itself, the rotational ratio on the drum becomes higher, thus reducing the overall gear ratio and, in effect, the pulling power of the winch. Winches are rated as to their maximum capacity at full amperage draw and on the bottom layer of cable. By the third or even fourth layer, the capacity is significantly less.

Actually, there is a lot more involved when picking a winch's capacity besides vehicle or pulling load weight. We can start out with angle or slope, or in simpler terms, how steep of a hill are you pulling the load up? Gravity is not our friend here, so vehicle weight is the first factor, then the slope. You have rolling resistance, starting with that which is inherent to the vehicle itself. Other factors are smooth surfaces, dirt or sand (wet or dry), as well as the different types of mud and their depths. The distance and angle of the pull also need to be taken into account. All these may present an instance where you need to use a factor of two times the load equals winch capacity.

There are several different formulas for factoring in these parameters that go beyond my desire to do that much math, but the 4x4 Icon (http://4x4icon.com/offroad/winch/index.htm#loads) has this way-cool spreadsheet that does it for you with your input of numbers.

For example: Pulling a 4,000-pound vehicle up a 50-degree slope that was in deep mud would take over 6,300 pounds of pulling force. On the other hand, if the vehicle was on a 10-degree slope on pavement, it would only take a pulling force of a bit over 800 pounds.

Actually, it's kind of fun to play with the different factors. It sure gives you a better idea of just how hard your winch has to work. One thing that the spreadsheet can't do for you is figure out how much pull is necessary when the tire encounters an obstacle such as a large rock. The size and shape of the obstacle, tire diameter, and even air pressure all have a cause-and-effect that's best figured by common sense and experience. Be sure to keep in mind that the figures on the spreadsheet may be higher, depending on the "stuck" condition of the vehicle, and are only to be taken as guidelines.

Now, let's go back a bit and assume you figure you can get by just fine with a 9,000-pound capacity winch. Remember what I said about the pulling capacity of the winch being diminished by the number of layers of cable on the winch drum? You really need to check and compare these actual capacities per layer with the figures that are provided by the particular winch manufacturer you're interested in.

Here is a simple example of how this works. Let's say you figure you want a winch with a 9000-pound capacity rated at the number-one bottom layer cable on the drum:

Cable layer 1 2 3 4
Capacity (1,000 lb) 9 8 7 6
Line speed (fpm) 6 7 8 9

From this simple chart, you can see that your 9,000-pound winch may only have a 6,000-pound capacity on the top layer, so on a short pull, your winch may not have the necessary capacity, especially on a pull in deep mud. On a long pull, you may run out of pulling capacity as the winch cable is spooled in, and you may have to relocate the pulling vehicle or your anchor point.

Something else to keep in mind is amp draw, as it has a major effect on winch performance. Amp draw is the amount of current that the winch motor requires to perform a load-pulling function. The harder the pull, the more amps are required, up to the point of the motor actually stalling as it reaches its maximum pulling power or capacity. Permanent-magnet motors draw less current than series-wound motors and can be built smaller. However, their drawback is that they- overheat quite quickly, so for serious long pulls without more frequent cool-down periods, the series-wound motors are generally the way to go.

Winches draw a lot of amps, some times in the 400-amp range under full load. So it is not hard to figure that a battery with a 55-amp rating doesn't have much reserve capacity. That's where our alternators come in. Keeping the battery charged to full capacity when doing a hard winch pull is nearly impossible, but because very seldom does one make a long continuous pull at full load, an alternator in the 100- to 150-amp capacity seems to work just fine.

Actually, you could overtax the alternator to the point of meltdown if you were doing a lot of winching without a cool-down period. But then again, that would mean a lot of winching. Yes, we could get into the fact that as the battery voltage gets drawn down, amp draw increases to meet the wattage demand, but we are not going to go that far. In reality, pick the best battery you can find within your financial range that will fit in your battery tray, and then keep in mind when the winch motor gets hot to the touch that it's time to give the motor and alternator some cool-down time.

Obviously, the more storage capacity of the battery or batteries and the larger output of the alternator, the more winching time one will have. When picking a winch, you might want to consider comparing amp draw and different loads. You just may find that a 10,000-pound winch pull draws fewer amps on, say, an 8,000-pound pull than a 8,000-pound winch does at full capacity.

Next month, hopefully I will get into proper rigging and ways to lessen the strain on your winch.

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