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FREQUENTLY ASKED QUESTIONS

A. Dashpots are mechanical piston/cylinder devices used to control velocity, dampen vibration, or absorb end of stroke shock. Generically, they are devices that dissipate energy as heat by forcing a fluid through through an orifice.

A. Dashpots exert forces because of pressure differences around the dashpot piston. The end of the piston where the rod attaches is always subjected to atmospheric pressure. The pressure on the other side of the piston can either be higher than atmospheric or a partial vacuum. In one direction of movement (push damping), an external force (gravity, springs, solenoids, etc) causes a mass (a slide, a lever arm, etc.) to start to move. With an air dashpot attached, the piston in the dashpot is forced to move towards the closed end of the cylinder, reducing the volume of air in the dashpot. Because the air can only escape through a tiny orifice, the pressure in the dashpot builds. The pressure will be higher if the external force is higher. In fact, the pressure will rise to the point where the dashpot force (pressure x piston area) equals the external force. In the other direction (pull damping), an external force (gravity, springs, solenoids, etc) causes a mass (a slide, a lever arm, etc.) to start to move. With an air dashpot attached, the piston in the dashpot is forced to move away from the closed end of the cylinder, increasing the volume of air in the dashpot. Because the air can only enter the volume through a tiny orifice, the pressure in the dashpot drops below atmospheric pressure. The pressure will be even lower if the piston moves faster and/or the orifice is smaller. Lower pressure means higher pressure differential and that means higher dashpot force (Force = pressure difference x piston area) .In fact, the pressure will drop to the point where the dashpot force (pressure x piston area) equals the external force. If a constant external force is applied to a mass, the dashpot force will equal the external force, causing a zero net force on the mass. This means that the acceleration on the mass is zero (a=F/m=0/m=0). The useful part of this a dashpot will cause the mass to move at a (nearly) constant velocity. And if the dashpot is adjustable, the velocity can be dialed in to the desired value.

 

A. The main difference is that our dashpots use air as the working fluid. A classical dashpot uses a liquid that is assumed to be incompressible, whereas air is compressible. A classical dashpot also exerts a force which is exactly linear with the velocity of the piston. Our dashpots are roughly linear with velocity, except at the start of the stroke, during vibration applications, and when the pressure in the dashpot approaches zero pressure (perfect vacuum) during pull damping.

A. The piston is made from carbon-graphite and the cylinder is made from borosilicate (pyrex) glass. The piston OD is ground to fit within a few ten thousandths of an inch to the cylinder. No seals are used. Here are some facts about these materials: 1) These materials have very low (1.8 x 10E-06 in/in/°C and 3.2 x 10E-06 in/in/°C respectively) thermal expansion, which allows for very close fits. 2) Have extremely low water absorption (high dimensional stability), which allows for very close fits. 3) The graphite from which the piston is made is a solid lubricant so there is never a need for additional lubrication. 4) The wear rate of the piston is very low for this combination of materials

A. Yes, glass is brittle. However, it is fairly strong with typical tensile strengths of 6-15 kpsi. Pressures in our dashpots rarely exceed 50 psi while cylinder burst pressures are in the range of 700-1000 psi. Also, glass has other great properties such as low thermal expansion, high dimensional stability, excellent surface finish, etc. Still, if you drop a glass dashpot on a concrete floor, it’ll break.

A. No. We choose to specialize in these two materials exclusively.

A. The friction coefficient is 0.2. Depending on the model, this converts to an absolute value from less than 1 gram to less than 8 grams.

A. Properly sized and implemented, Airpot dashpots are capable of hundreds of millions of cycles. Wear of the connecting rod joints will usually be the determining factor and actual values will vary with applied force and rod angle. However, in most cases, the Airpot can outlast the machine in which it is used.

A. Yes. This will obviously greatly increase the damping capability. However, it is usually best to submerge the entire dashpot in the damping liquid. This prevents the liquid from drying on the inner surface of the cylinder and leaving a residue that could interfere with piston travel. Airpot actuators are sometimes used for pumping liquids. Here also, it is advisable to keep any liquid from being allowed to dry on the inner cylinder wall.

A. The S is for standard conditions. We test at 50 psig and convert the volumetric leak rate (at 50 psig) to a volumetric leak rate at atmospheric pressure.

 

A. This will depend on the rate of force rise desired and the maximum force required. Please review the damping curves shown under here.

A. Yes. We make three general types: two way damping units, push damping units, and pull damping units. A push damping unit damps in the push or retraction direction and has almost zero resistance (damping and friction) in the extension direction. A pull damping unit damps in the pull or extension direction and has almost zero resistance (damping and friction) in the retraction direction. We accompish this by building a check valve into the piston assembly. In one direction of travel, air must flow through the needle valve, but in the free direction, air can flow freely through the check valve so that a pressure difference around the piston doesn’t develop.

A. The ideal range is from 0 to approx. 30 lbs. depending on model and damping direction. Push damping units in all models are capable of higher maximum forces than the optimal rating but the additional stroke lost to gain the added compressive force may leave insufficient stroke for useful damping.

A. The maximum pull force is not the force at which the rod or piston will break. The maximum pull force is simply the maximum damping force that a particular dashpot can exert in the pull or extension direction. Here’s why. First, there is always atmospheric pressure (14.7 psia) on the rod side of the piston. Next, pulling on the dashpot rod generates a partial vacuum on the other side of the piston, the piston face. Pulling fast on the rod with a small or no orifice will reduce the pressure to be close to a perfect vacuum (0 psia). However, the pressure can never equal or go below 0 psia no matter how fast or how hard you pull on the rod! The max pull force is then the maximum pressure differential multiplied by the piston area. For the model 2K160, the max pull force = 14.7 psia x .309 in^2 = 4.5 lbf.

A. Let’s use an example to explain. A model 2K240 has a max pull force of 14.7 x .701 = 10.3 lbf. If you had a 20.6 lbm object, earth’s gravity would cause this object to weigh 20.6 lbf. If you dropped it, it would accelerate downward at 20.6 lbf / 20.6 lbm = 1g (32.2 ft/s^2), neglecting air resistance. If the object were dropped again with a model 2K240 pull dashpot attached, the dashpot would resist with 10.3 lbf. The net force on the object would be 20.6 – 10.3 = 10.3 lbf. The acceleration downward would then be 10.3 / 20.6 = 0.5g. By the way, you would not want to use a model 2K240 in this case. The reason for using a dashpot in this case would be to control the velocity of a mass. A mass that is still accelerating is not controlled. Use a model 2K444.

A. We measure the time it takes a known mass to drop vertically under the force of gravity through a known distance while being damped by our dashpot. We functionally test every unit to insure that it at least meets our specified damping capability with the orifice closed. Most of our units actually exceed the published specification by varying degrees.

A. On all models smaller than our Model 325, the underside of the plastic adjustment knob contains a hex recess which loosely engages the hex head adjusting screw. As the knob is turned, the screw has room to move up and down within the knob’s hex recess. The knob snaps onto and rotates on the specially shaped radius shoulder nut that is used to mount the dashpot to the mounting surface.

A. Perhaps. We do not currently have a built-in design for this feature but there are ways of achieving this using small check valves with our remote adjustment orifices connected to the Airpot. Please speak with an Airpot applications engineer if this is an important need for you.

A. Yes. We have the ability to provide a specified damping range by using selective fits between the piston and cylinder or scribing the piston to provide a leak path with a known leak rate. There are limitations to this feature however and use of this capability will require discussion with one of our application engineers.

A. No, the piston will be damaged if you use it as a stop. The dashpot must be designed in such that there is clearance between the piston and the end stop at each end of travel.

A. It is possible in most units. However, we do not recommend it. Once the piston leaves the cylinder it is vulnerable to distortion, damage, contamination, and being misfit into the wrong cylinder (pistons and cylinders are selectively matched and fit to tolerances measured at the .xxxx level of accuracy).

A. Yes, and they’re inexpensive. Please click here to see our piston-cylinder selections.

A. Yes. We do offer these with minimum order requirements. Please contact our sales department.

A. Yes. We do offer these with minimum order requirements. Please contact our sales department.

A. No. However, our dashpots may often still be used effectively on small swinging lids and rotating levers and links.

A. We currently have six bore sizes ranging from .220 inches to 1.750 inches. Please click here to see our dashpots selections.

A. Yes. We very frequently customize to suit individual requirements. However, we do not offer custom bore sizes unless the quantities are very high.

A. Yes. Please contact our application engineering department.

A. Call us toll free at 1-800-848-7681 and ask to speak with an application engineer, or email engineering@airpot.com.

A. Most engineers seem to like the benefits and flexibility of selecting customized units from our wide range of options. This makes it somewhat impractical for USA distributors to stock the enormous range of possible configurations. We do sell dashpots through distributors outside the USA who order for resale primarily after they have received a firm customer order for customized units. For lower volume applications we do have a stock line of configurations which we sell direct in the USA and through our overseas distributors.

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A. Incredibly lower friction, ultra low pressure response capabilities, and ability to operate over a much broader and extreme temperature range with no performance changes. These properties are the result of a unique construction that eliminates the need for interference fit seals and that uses thermally stable, self lubricating materials.

A. We define stiction as the irregular motion that results from the difference between static (at rest) friction and dynamic (running) friction. Stiction is manifested by hysteresis, jerky starts and inconsistent velocity throughout the stroke of a mechanism. Anti-stiction® means construction designed to avoid these problems and provide a super smooth start, consistent velocity, and ultra low hysteresis.

A. The piston is made from carbon-graphite and the outer steel cylinder is lined with an inner cylinder made from borosilicate (pyrex) glass. The piston OD is ground to fit within a few ten thousandths of an inch to the inner cylinder. No seals are used. Here are some facts about these materials: 1) These materials have very low (1.8 x 10E-06 in/in/°C and 3.2 x 10E-06 in/in/°C respectively) thermal expansion, which allows for very close fits. 2) Have extremely low water absorption (high dimensional stability), which allows for very close fits. 3) The graphite from which the piston is made is a solid lubricant so there is never a need for additional lubrication. 4) The wear rate of the piston is very low for this combination of materials.

A. Yes, glass is brittle. However, it is fairly strong with typical tensile strengths of 6-15 kpsi. Our cylinders are conservatively rated at 100 psi while cylinder burst pressures are in the range of 700-1000 psi. Also, glass has other great properties such as low thermal expansion, high dimensional stability, excellent surface finish, etc. The outer steel sleeve helps protect the glass from harmful blows, although if you drop an Airpel on a hard floor, it is susceptible to breakage.

A. No, never. It is recommended that you use clean (5 micron filtered), dry, oil free air.

A. The friction coefficient of the piston / cylinder is 0.2. Depending on the model, this converts to an absolute value from less than 1 gram to less than 8 grams. Under pressure, the friction will normally run at approximately 2% of the load. Most units can respond to pressures as low as 0.2 psi.

A. Properly mounted and implemented, Airpels are capable of millions of cycles. Side load will have the greatest influence on wear and will normally be the determining influence on cylce life.

A. Yes if they are clean and not allowed to dry out, leaving a residue.

A. No. We choose to specialize in these two materials exclusively.

A. The S is for standard conditions. We test at 50 psig and convert the volumetric leak rate (at 50 psig) to a volumetric leak rate at atmospheric pressure.

 

A. No, the piston or the inner cylinder can be damaged if you use it as a stop. You will need external stops on your mechanism and the Airpel should be mounted such that there is at least 1mm clearance between the piston and the end stop at each end of travel.

A. Yes. However, you should take into account that since the Airpel does not use interference fit seals, it will slowly leak air. Thus, the holding force will diminish unless air pressure is provided continuously.

A. We currently have four sizes; 0.366 in, 0.627 in, 0.945 in, and 1.281 in.

A. Yes. We routinely provide custom strokes and rod lengths. However, we do not offer custom bore sizes.

A. Both. Our goal is to see that you get what you need when you need it. We will recommend a distributor if we have one serving your area, but if they are out of stock or if you do not wish to purchase from them for any reason, it is our pleasure to serve you directly. For a list of our distributors, please click on one of the Airpel Distributor drop down links in the Reps link at the top of the screen.

A. Call us toll free at 1-800-848-7681 and ask to speak with an application engineer, or email engineering@airpot.com.

A. Yes. We accept Visa, MasterCard and American Express.

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