Something I’ve always wondered- I’m shopping for a new garage door opener. Everyone I talk to asks me how large the door is. What difference does that make? Every garage door I’ve ever seen has springs, so any man woman or child can lift it. Therefore, even the lowest HP opener should work just fine on any door. Why do they recommend higher HP on larger doors?
just curious-
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Replies
In theory a more powerful motor is needed to accelerate a heavier door, even when spring balanced.
In practice, though, a good part of it is just to try to sell you an SUV, when you'd be perfectly happy with a Prius.
OK, now we're talking physics. Given two garage doors- one weighs 100 pounds, one weighs 300 pounds. Both have springs such that the effictive weight one must lift to open either door is 30 pounds. Does the heavier door have more inertia to overcome to get it moving? Intuitively, I think not, as the effective weight is the same. Therefore same HP opener will work the same.Perhaps, when the door is fully up, and the springs aren't contributing anything to closing the door is when more power would be required to get the door moving.Bill
One of the basic laws of physics:F = M * AImagine a giant semi loaded with lead ingots on a perfectly smooth road and with frictionless bearings and tires. Now imagine a tiny circus clown car on the same road with the same bearings/tires.Which one will be easier to start moving?
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
Obviously, but you're comparing two different masses (weights). In the garage door example, aren't the springs equalizing (at least for lifting) the weights of the doors?
They're equalizing the force required to overcome gravity, but not the force required to accelerate the door from a standstill to a moving state - nor the force required to decelerate and stop it at the end of its travel.Rebeccah
is the spring not contributing a portion of the force necessary to start moving the mass? It seems to me that the spring would contribute sufficient force that the opener would only "see" the minimum weight of the door.
This is my logic, too, however, I see how stopping the door at the top will be different, as the heavier door is not just heavier, but now the springs are also trying to lift the door, so you have to stop the door plus the force of the springs.
It takes the same force (well, actually momemtum) to start something moving as to stop it.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
OK, get two doors, properly balanced, one heavy, one light. Slam both on your foot. Which one hurts more?
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
Slam both on your foot. Which one hurts more?
The one I slammed the doors on.
> Imagine a giant semi loaded with lead ingots ....
Better yet, one you can actually do. Try a tugboat or commercial fishing boat tied up at the dock -- dozens to low hundreds of tons. One guy can get it moving, but it takes some time. A friend of mine actually did that once, in order to shift the lines and make room for the next boat. Scared the crap out of some folks in a restaurant, the boat was headed right at them.
-- J.S.
A quarter horsepower door opener will reliably open a two car door if the door is properly adjusted and balanced. More power is a waste of money.
A more massive door will have more inertia, but doesn't necessarily need a bigger opener. The springs store energy and release it when they move. So the additional energy needed to accelerate a more massive door is provided by energy stored in the springs and the opener does not need to be any stronger.
Power = force x velocity. Regardless of the mass of the doors, if the springs are adjusted to require an equal lifting force, and the openers operate at the same speed, then the power needed by the openers are equal. The power is the same if both the force and the velocity are the same.
Salesmen recommend more power because it is a more expensive unit, but it can also tolerate less fussiness in the balance of the door.
Okay...I started this thread questioning the assertion that more HP is needed for a larger door. After this discussion, I now think it IS needed, because CLOSING the door (where the springs can't help) and stopping the door at the top is harder for a larger door.Thanks.
I think you can forget your argument about needing more power to stop a large door, since the motor does not stop your garage door anyway. Garage door openers employ a worm gear to move the door, either directly with a long worm gear or indirectly in their reduction gears, so the door stops when the motor ceases turning, no matter how powerful or how puny the motor.As for the power required to start a heavy door moving, the gear ratio is so high on those worm drives that the mechanical advantage is huge. Consider the jack for a pickup or van. With very little effort I can raise half a ton or more of vehicle weight using a very simple worm-gear jack.BruceT
Edited 11/15/2006 4:51 pm ET by BruceT999
Boooogus!Think about it: You've got two doors: one, perfectly balanced, weighing a ton, and another, perfectly balanced, weighing an ounce. Which will be easier to move?
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
Well, I understand, but does 'perfectly balanced' mean the force required to get the door moving when it's in the 'up' position is equal, no matter what the door weighs? I think not. It would be quite a bit harder to get the heavy door moving. Once it's decending, and the springs 'perfectly balance' the force of gravity against the weight of the door, all should be equal.
Well-designed door hardware will "balance" the door across the entire range of its motion.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
No, geomentry makes that impossible, at least at a reasonable cost.A balance door will stay in place if opened about 1/2 way.Open more and it will continue to open.Lower it some and it will continue to lower.
Not true, for a segmented door that runs up onto a horizontal track. As the spring shortens up and loses force, more and more of the door is horizontal, maintaining a near-perfect balance.And even lacking the horizontal track there are tricks (such as variable-diameter cable spools on a torsion door) to allow constant force.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
They do on torsion spring doors.
(Not that many doors work out that way. It's possible, but it takes more thought than goes into your typical Menards special.)
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
My Sears opener with a 1/2hp motor (chain drive) has worked fine for 20 years on a double wide steel door. (knock on wood) I have adjusted the tension in the chain a couple of times. The chains stretches a little over time.
PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood
Homer Simpson taught me that:
Door goes up, door goes down.
Door goes up, door goes down.
Gotcha beat..Just replaced a 23 yr old 1/3 hp Sears on 16' WOOD door. Very heavy. Probably the last wood door left in entire neighborhood.I did put in 1/2hp simply because the extras that came with the step up. Also got the screw instead of chain.Can't think with all the cheap tin doors about anything would lift and close them.Pete in Plano
You guys are way over thinking this.
You have two doors. Door #1 weighs 100 pounds. It is balanced by a spring so that the force to lift it is 10 pounds. Door #2 weighs 800 pounds. It is balanced by a spring so that the force to lift it is 10 pounds. A garage door opener is connected to each door. The force and power expended by both garage door openers is the same. The force is 10 pounds. The power is the force times the velocity. If identical openers are hooked to each door then they are geared to operate at the same speed. Since the force is the same and the velocity is the same, so the power is the same.
Force = mass x acceleration.If the spring is tuned to *hold* the door against gravity, that is one thing, and is how I always conceptualized the operation of the balancing springs. That is, the springs apply a force equal to the door's mass x g, the acceleration due to gravity (at least with the door in a certain position).If you want to accelerate the door from a standstill to some velocity over a period of a second or two, that's *additional* force that you need to apply. That additional force is proportional to the mass of the door as well as to the acceleration with which you want to open it.From the posts I see here, it is clear that others have a different idea of how the springs are tuned. Suppose they are tuned so that the amount of force required to accelerate the door is independent of the mass of the door.g = acceleration due to gravity
a = acceleration required to get the door moving from 0 to the desired speed within the desired amount of time
m = the mass of the door
F1 = the force required to hold the door against gravity
F2 = the force required to accelerate the door to a desired speed within the desired amount of time
Fd = the desired force you need to supply to open the door
Fs = the force supplied by the springsFd = F1 + F2 - Fs
= m*g + m*a - Fs
Fs = m*g + m*a - FdIf you are hoping for some help from the springs to fight inertia, then you want
Fd < m*a
Which means that Fs > m*g, and the door will tend to rise on its own. I don't think that's what garage door springs do, ordinarily.Rebeccah
Well, no, that's not correct. The mass of the door doesn't go away because a spring is acting on it.
-- J.S.
"You've got two doors: one, perfectly balanced, weighing a ton, and another, perfectly balanced, weighing an ounce. Which will be easier to move?"
That sounds suspiciously like the argument that a big heavy stone will fall faster than a small one. Thanks to Galileo we know that assertion to be false.
If the springs are set such that the door requires 10 pounds of pressure to overcome inertia and gravity, then it shouldn't matter what the original weight of the door was, the important measure is the 10 pounds.
If what you say were true then a mechanical watch wouldn't work.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
Don't understand your assertion. Why wouldn't the mechanical watch work if the springs on a garage door require 10 pounds of pressure? :)
I'm no expert on springs and things, but the spring needs to be sized to the job. If the spring on the door is set to require 10 pounds of pressure to get things moving, then that is what it takes - it isn't adjusted to release the energy without the input of pressure. The spring on the watch is set to release it's potential energy as kinetic energy for the rest of the workings, and it releases that energy without additional input because that is how it is sized and adjusted. Each spring is adjusted differently for a different purpose and storage/release of energy.
The best explanation I can think of is this:When you hit the grocery store you grab a shopping cart. It's pretty easy to push, since it's empty an weights maybe 10# or so. Starting and stopping it are no big deal, since it's light.(You don't need springs to hold it up, since it rests on a level floor)Now put 30# of groceries in it. You can still push it to get it rolling and pull back to stop it - But it takes more effort because the cart + groceries weigh 4 times as much.The floor still supports the cart - But it's harder to start and stop the cart because it has more inertia. Same with a garage door - The springs hold the door up. But it takes more effort to get a heavy door moving than a light one.
Bumpersticker: Back off! I'm not that kind of car.
You are assuming the same springs for both doors, I am not.
I've seen doors that literally weighed 1/2 ton that I could open and close with my little finger. They open and close easier than the doors in my house. Why? Because they are adjusted better than the doors in my house.
I explained it the best way I know how. Don't know what else to say.
I think on-stage nudity is disgusting, shameful and damaging to all things American. But if I were 22 with a great body, it would be artistic, tasteful, patriotic and a progressive religious experience. [Shelley Winters]
You were adding mass to an otherwise closed system, so you were not explaining the point I was trying to make.
Say you have a door that weighs 100 pounds and a door that weighs 10 pounds, and an acceleration of 1 foot per second. Force = Mass * Acceleration, so your force for the heavy door is 100 and the light door is 10. Great, but you want the force for the heavy door to be 10 so you introduce a spring to make F=10. But your acceleration is still 1 foot per second. What has to change to make this work? The Mass. The spring is in essence lightening the door to allow reduced force to be used.
You're trying to say the springs offset the MASS of the door. In reality, they offset the WEIGHT of the door.
The other day I slammed a finger in the car door. My friend sure was pissed.
"You're trying to say the springs offset the MASS of the door.
In reality, they offset the WEIGHT of the door."
You're kidding, right?
In case you are not, I'll a throw a little wikipedia back at you:
http://en.wikipedia.org/wiki/Weight
"..weight is a force that results from the action of gravity on matter". In other words, Weight = Mass * Gravity, (aka Force = Mass * Acceleration).
Just to illustrate the point I'm trying to make - What would happen if you installed a garage door in space, where it was weightless?It would STILL take force to make the door start moving and stop moving, because it has the same mass even if it's weightless.Weight and mass are 2 different concepts.
To have that sense of one's intrinsic worth... is potentially to have everything... [Joan Didion]
To: aimless
In your example you say mass x acceleration and then say 10 feet per second. Thats velocity i.e. feet (distance) per second (time) not acceleration i.e. feet (distance) per second (time) per second (time) or feet / second^2. You are right about velocity. Thus the door moves at a nearly constant speed after it starts moving and the spring provides enough force to overcome frictional losses. But to start the door moving a bigger door requires a bigger motor.
JH
All I know is: With everything (lift) disconnected, when I reach down to open (lift) the door, IT'S HEAVY..... but once I get it about a foot high, the spring magically makes it pretty easy to continue to lift to full height.
I suspect the lift has to be sized to overcome the initial inertia. A tiny lift could keep it going once started.
I'd go with the larger lift motor based on my elementary findings. Just my opinion, but it makes sense to me.
You're saying that if the spring offsets the force, the mass doesn't matter, from the standpoint of acceleration. If that were the case a balance-wheel watch would not work.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
If you have a spring offseting the force, then it is also offseting either the mass or the acceleration, or you don't have a balanced equation of F=M*A. If you've reduced F, you are either reducing M or you are reducing A. So, if you are using a spring to make a big heavy door require less force to get your acceleration, then you are in essence reducing the mass of that door.
Wikipedia has a page on inertia which might help you understand it:http://en.wikipedia.org/wiki/Inertia
Never knock on Death's door. Ring the bell and run, he hates that.
All I can tell you is that I sleep THAT much through three years of college physics. You are wrong, but it's pointless to try to convince you.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
In my experience, the larger horsepower machines are put together a little better.
"In my experience, the larger horsepower machines are put together a little better."
That's what I was thinking.
The cheapest openers are built with the smallest, cheapest motor they can get away with.
The better ones have heavier duty motors and other parts.
If I see any man turn his back today, I will shoot him through. I have two pistols loaded. But I will not ask any man to go further than I do. I will fight as long as I have a leg or an arm. [George Washington, speech to his troops before the Battle of Long Island, 1776]
You know, all the physics calculations in the world are irrelevant unless you know the work that the motor is capable of doing. Whatever the mass of the door, the balancing point, etc., if a smaller motor of equal quality can do the job, the larger one is unnecessary. To reduce it to the absurd, there is absolutely no doubt that an old 440 cu inch auto engine, properly geared, can power a garden tractor. However, a 22hp engine can do the same job admirably, so why bother with the 440?I have an old Sears 1/2 hp opener that has pulled a heavy segmented double-width metal overhead door up and shoved it back down again several times a day with no trouble at all for 14 years, despite some warping on both the door and track that cause sticking.Why would I want more?
You're trying to make a point by comparing something radically different than what we're talking about. Kinda like my Wife does.A motor that's just BARELY big enough or even a hair undersized isn't gonna last as long as a motor that's just a little bigger than it needs to be.
If you can't change your mind, are you sure you still have one?
To reduce it to the absurd, there is absolutely no doubt that an old 440 cu inch auto engine, properly geared, can power a garden tractor. However, a 22hp engine can do the same job admirably, so why bother with the 440?
Bragging rights. -:)
True. Or you might want to win a garden tractor pulling contest. Or maybe you lost your license for street vehicles and still need to get around in a hurry. I suppose if you look at it in just the right twisted way there are several reasons for having a 440 garden tractor.
BTW, nice to see that someone actually read my post. I thought it got lost in completely specious arguents about physics. I wonder if the poor person who posted the original question thinks he or she was accidentally redirected to a web site for certified mental patients.
From personal experience, I found the 426Hemi better suited to garage door duties. The 340 six-pack, otoh, yields way too much acceleration - you'd spend a fortune on pulleys and belts gearing it up to door velocities that fall within the door mfg's specs.
"The cheapest openers are built with the smallest, cheapest motor they can get away with.The better ones have heavier duty motors and other parts."Kinda, sorta. But you need know what you are comparing it against.My old opener was a Liftmaster 1/3 hp. The Liftmaster is a pro version (one piece rail) made by Chamberlain.I have an 18 ft wood door that is HEAVY and often unbalanced. It was still working when I replaced after 25 years, but was on it's last legs.But the 1/3 hp that I have seen at home horror stores have bee cheaply made.
We've got an old Sears unit -- either 1/4 or 1/3 HP -- that's 30 years old. It's on a similarly heavy hardboard two-car door. Had the time delay for the light go out once about 20 years ago, and the clickers broke so I added an after-market transmitter/receiver kit, but otherwise the thing's had essentially zero service.The door itself, however, is on its last legs. The rollers are all worn out and no one makes that size any more.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
More power is always better!
Who'd've thought there'd be so many posts on the subject!
To answer the question in the original poster's question, it does not make a difference.
Lots of people mentioned mass. Mass is irrelevant to the amount of power needed to move the door. Power = force x velocity. There is no mass in the equation. The power depends only on the force and the velocity.
In a garage door, part of the power needed to move the door comes from the spring and part comes from the opener. If two doors are adjusted to require the same lifting force then the power needed to move them is the same regardless of their mass.
That last statemnet is wrong dude, if a door cannot lift itself with the springs and your hand the springs are not big enough. Recipe for replacing an opener early.
You're wrong, of course, but the difference is probably negligible, unless you want to open the door real fast.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
If you have made it this far, thanks for reading - nice stamina
My garage door is 18x9 (big), and opens readily with the 1/4 hp opener.