Cable Cylinders
Cable cylinders are pneumatic air actuators (cylinders). They are rodless air cylinders, but don't use a band type sealing method as band cylinders do.
Rather than having a piston rod sticking out one or both ends of the air cylinder like standard rodded air cylinders, cable cylinders use a cable which enters either end of the cylinder barrel, and is attached to either side of the internal piston.
The cable exits the cylinder barrel through the cylinder end caps, winds around a sheave or pulley on either end of the cylinder barrel, and joins outside the cylinder barrel in a non-supported carriage.
When compressed air enters the cylinder the piston moves from end-to-end. The cables which are attached to either side of the piston and extend out the ends of the cylinder, move as well. Depending on the direction of the piston both the carriage and any on-carriage tooling moves towards one end of the air cylinder or the other.
Cable Cylinders Save Space
The carriage moves within the length of the cylinder barrel. The benefit of this is space saving.A typical NFPA type cylinder has a cylinder rod that protrudes from one or either end of the cylinder. The overall length of the cylinder will more than double the length of that rod. A 12" NFPA cylinder will require an installation footprint of about 28" depending on the manufacturer. A 12" cable cylinder may need only 16" footprint.
Through use of creative cabling, the cable cylinder allows the carriage that is being driven by that cylinder to be located some distance from the actual barrel and piston, unlike almost any other kind of air cylinder.
Another significant benefit of a cable-type air cylinder is it's relatively low cost per inch of stroke.
Let's take a look at some of the items that make up a basic cable cylinder. Of course, each manufacturer will have their own styles and design differences. The drawing just below will give you a good idea of the concept.
Item 1
The barrel of the cable cylinder is usually round, though, as long as the piston would move easily, there's no reason why it couldn't be elliptical or octagonal.Barrels can be made of steel, aluminum or composite materials and can be many feet in length. As the length of the barrel increases, this creates a need to provide barrel supports to ensure that the cylinder barrel stays straight.
The cable cylinder manufacturer will provide load characteristics for their cylinder.
A drawback of the typical cable cylinder barrel is that it does not lend itself to attaching proximity switches easily. Some manufacturers have overcome this by creative design.
Item 2
This is an end cap in the center of which is a hole through which the cable passes from the end of the cylinder.The cable will pass through the hole, and seals in the end around the cable will help ensure that compressed air doesn't escape easily.
The end caps can be cushioned to decelerate the piston and the load. Depending on the load however, it may be better to have the carriage and load stopped externally with shock absorbers or a fixed stop, than using in-cylinder cushions.
Item 3
This is the sheave or pulley which is attached to the end cap on both ends of the cable cylinder. The cable, attached to both sides of the piston inside the cylinder, travels through the end cap gland seals, around the sheave, and is ultimately attached to a carriage.Item 4
This depicts the cable. One of the significant benefits of the cable cylinder is that the cable can be relocated elsewhere via a series of sheaves, and, when the piston inside the cable cylinder barrel moves with incoming compressed air, the carriage will move regardless of how far away from the cylinder it travels. The carriage travel could even move tangentially to the orientation of the cylinder itself.Please see the picture below to better understand the concept.
Depending on where extra pulleys are placed the weight moving capacity of the cable cylinder can be increased or lessened, and the carriage travel distance can be lengthened or shortened even as the piston strokes full length inside the barrel.
Item 5
By the design of the cable cylinder, the carriage typically is unsupported. Therefore, the use of rails and bearings to carry the load, and the use of the cable cylinder carriage to provide only linear motion, is normal.
If the cost of providing a carriage support is higher than expected, consider using a band cylinder which, depending on the bore diameter, has integral support.
The cable cylinder manufacturer will provide load characteristics for their cylinder.
Item 6
This depicts the piston. Different manufacturers offer different piston designs, yet all will have piston seals, an 'O' ring or 'D" ring, or multiple rings around the circumference of the piston.There may also be a magnet to allow piston position sensing, though, as mentioned before, you will need to get creative as to where to position the prox switches.
Selecting A Cable Cylinder
What do you need to know to select the appropriate cable cylinder? Here is a check list:- What is the weight and size of the load to be moved
- Where will the load be in relation to the center of the carriage and/or to the carriage itself
- What is the distance the load will be moved
- What is the speed required in distance per second
- What will stop the load / carriage at the end of stroke
- Is position sensing required
- How will the cylinder be mounted
This will ensure that you won't get 'cable slap', extraordinary gland seal wear, and lag time between the piston starting to move, and the carriage starting to move.
