Posted by Danny Basu on Wed, Dec 07, 2011 @ 06:11 PM
Medical device, aerospace and automation manufacturers often require flexible cables for their products. Flexible wire and cable can have many interpretations. Sometimes a simple description such as, “the wire needs to be as limp as a wet noodle” is enough to understand your needs. The choice of type of conductor and insulation can greatly influence the flexibility of the cable performance. Understanding what the flexibility requirements are will help to determine the materials and construction for the flexibility of the wire or cable.
When specifying a flexible cable you need tell your cable manufacturer what the intended use will be.
- Do you need to route it through equipment or a tight space?
- Will it be subject to repetitive flexing?
- Do you need the cable to flexible during use?
There are several factors which make a cable flexible, a few are:
- Conductor stranding
- Shield types
- Cable lay
- Insulation materials.
Typically in conductor stranding, the more strands a conductor has the more flexible the wire will be. Normally “off the shelf” wire and cable is stranded in 7 or 19 strand configurations, is not flexible enough for most applications requiring flexible or high flex cycles. The general rule is the higher the strand count of the conductor, the more flexible the wire or cable will be. Stranded conductors are composed of un-insulated “strands” of wire twisted together. The advantages of stranded conductor over a single strand are increased flexibility and flex-fatigue life. When you see 22AWG (19/34) for example, what the description means is that there are 19 strands of 34 AWG making up the 22 AWG conductor. Other common options for a 22 AWG is a single strand (solid) up to 168/44 strands. The construction of the conductor also plays a role in the cable’s flexibility, for example a rope stranding is the most flexible. For repetitive flexing applications such as robotics and automation, the use of high strand alloy material is recommended.
Choosing the right insulation can add to the cable’s flexibility. Silicone is one the most flexible of the compounds used. Silicone wire is used to meet a variety of demands such as extreme high and low temperature requirements, flame resistance, flexibility, strength and purity. The application and environment also play a role when choosing jacket material. Harsh environments, chemicals, and abrasion will narrow down your insulation and jacket options. Discussing your flexible wire application with a cable manufacturer can help you choose the right materials for the performance and longevity of your product.
For assistance with custom wire & cable design, contact a design expert at
Calmont Wire & Cable, Inc.
420 East Alton Ave.
Santa Ana, CA 92707
Website: www.calmont.com
Email: calsales@calmont.com
Phone: 800-905-7161 ext. 135
Posted by Danny Basu on Wed, Nov 09, 2011 @ 02:45 PM
To make flexible wire, Calmont starts with many strands of fine wire that are equal to various American Wire Gage (AWG) sizes. The conductors can be made with bare, tin plated or bare copper and high strength copper alloys. The insulation, shields and jackets are made with flexible materials.
Superflex Wires use flexible PVC for the insulation. When two or more conductors are cabled together and jacketed with flexible PVC, we refer to that as Superflex Cable. The cables can contain shields of Aluminized Mylar tape with a drain wire, spirally wrapped copper or copper alloys or braided with copper or copper alloys.
See our Superflex Wire page for the list of conductor sizes usually made. We offer other sizes not shown on the list by request. For a list of the common AWG sizes visit our Wire Card page.
For other insulations and jackets please visit the Calmont Triangle to learn about the materials and methods we use to manufacture our wires and cables.
For assistance with custom wire & cable design, contact a design expert at
Calmont Wire & Cable, Inc.
420 East Alton Ave.
Santa Ana, CA 92707
Website: www.calmont.com
Email: calsales@calmont.com
Phone: 800-905-7161 ext. 135
Posted by Danny Basu on Tue, Oct 04, 2011 @ 03:11 PM
Medical Hand Piece Cables
Cables for medical hand pieces must carry the power, control, sensor and data to and from the hand piece. The cable may be as simple as a two-conductor cable, providing power to a cut & cauterize scalpel or a multi-conductor power, control and sensor cable used on an orthopedic surgery device. Cables for handheld medical lasers likely contain lumens for cooling. Ablating hand piece cables can also contain lumens for irrigation and suction removal.
The design of medical hand pieces falls into two categories: re-useable and disposable. See below for information on Disposable Medical Cables and Re-useable Medical Cables.
Disposable Medical Cables
Disposable medical cables are made from the lowest cost materials that will meet the design needs of the device.
Conductor cost is dictated more by the number of strands used to make the conductor. The higher the strand count the more flexible the wire becomes. A marketing trade-off is often made for disposable cables, i.e. increase stiffness can be tolerated by the need to hold cost down. 7 or 19 strand conductors can be used, where in re-useable cables the strand count can be well over 50. The use of platings is reviewed and bare copper is the most often used. Tin plating is used only to enhance solderability. Crimping the wires is often the cheapest means of terminating the conductor. Silver plating is rarely used for disposable cables. Conductor sizes are determined by the current needs of the device.
Insulation and jacket materials are chosen by cost. Low cost materials such as Poly Vinyl Chloride (PVC) or Thermo Plastic Elastomers such as poly urethane are often used. Polyethylene can be used as insulation. Insulation and jacket thickness are determined by the voltage ratings of the signals in the cable.
Shields, when required, are usually of a spiral type rather than braid to hold the cost down. Spiral shields may lose shield effectiveness during repeated flexing, but the number of flexes for disposable cables is limited.
Jacket colors usually are chosen by the program. The need for gamma sterilization may limit the color shades available. Colors can change during gamma sterilization and are usually required to be stable for less than 10 cycles to allow for re-sterilization when repackaging is needed.
Re-useable Medical Cables.
Medical cables designed for re-useable devices have to use materials that will withstand several cycles of sterilization by alcohol based agents or autoclaves.
Conductors are made from high strand count conductors for maximum flexibility and often they are made from high strength copper alloys. The size of the conductors is determined by the current they must carry to the device. To withstand the rigors of sterilization, plating’s of tin or silver platings may be required.
Insulations must also withstand the rigors of sterilization and are often either Teflon’s such as FEP, PFA or Tefzel® or Silicone Rubber. The use of lower cost materials may be made on a case by case basis.
When shields are required, they must be made from materials resistant to many sterilization cycles and are often silver plated copper or silver plated high strength copper alloys. Shield construction is usually of a braid rather than a spiral, as spiral shields can move during flexing and compromise the shielding effectiveness.
The cable jackets are made from Santoprene™ TPE which is good for up to 100 cycles of autoclaving, or Silicone Rubber.
For assistance with custom wire & cable design, contact a design expert at
Calmont Wire & Cable, Inc.
420 East Alton Ave.
Santa Ana, CA 92707
Website: www.calmont.com
Email: calsales@calmont.com
Phone: 800-905-7161 ext. 135
Posted by Danny Basu on Tue, Oct 04, 2011 @ 12:36 PM
Cables for Aerospace applications require light weight constructions. For space applications, the insulations and jackets must have low out-gassing capabilities. Conductors must be silver or nickel plated. Tin is no longer allowed for flight and space applications. The use of high strength alloys is required for wires smaller than 26 AWG. Some Aerospace Cable applications may still allow for 19 strand conductors but most require the use of higher strand count for flexibility.
The Aerospace Wire and Cable insulations must have low out-gassing properties, and this eliminates many materials such as PVC. Common insulations are Teflon’s such as FEP, PFA and Tefzel®. Standard silicone rubber cannot be used in space without a waiver and significant post baking is needed. Calmont now has a low out-gassing silicone rubber that meets the NASA out-gassing requirements. Light weight shield materials such as Aracon® are available for reducing the weight of shields.
For assistance with custom wire & cable design, contact a design expert at
Calmont Wire & Cable, Inc.
420 East Alton Ave.
Santa Ana, CA 92707
Website: www.calmont.com
Email: calsales@calmont.com
Phone: 800-905-7161 ext. 135
Posted by Barbara Monteleone on Thu, Mar 10, 2011 @ 04:34 PM
SHIELDED CABLES: WHY SHOULD I SHIELD MY CABLE?
Purpose of a shield: To Prevent interference from entering or emanating from a cable.
The three types of interference are:
Radio Frequency Interference (RFI)
Electro-Magnetic Interference (EMI)
Electro-Magnetic Pulse (EMP)
RFI was the earliest interference engineers had to deal with. Early radio signals easily found their way into devices. Coaxial cables and shielded twisted pairs using copper can eliminate most of this type of interference.
EMI comes more into play today with high power transmission lines, higher magnetic fields, such as MRI machines in hospitals, and other high power applications. Copper may not offer much resistance to higher magnetic fields; therefore the use of magnetic materials such as high permeable irons may be required.
EMP is produced by the detonation of nuclear devices. When the Hydrogen Bombs were tested in the 1950s at Bimini Atoll in the south pacific, circuit breakers at power stations in Hawaii were tripped by the EMP wave from the detonation. Critical military and civilian circuits have to be protected from an EMP condition. These shields require the use of both high and low permeable materials to reduce the effect of an EMP.
Types of shields in use today:
The simplest is a plated plastic film (aluminized mylar) wrapped around a cable or twisted pair. A drain wire contacts the foil along the cable to maintain a low resistance.
For increased shield effectiveness, a loose braid may be placed over the tape. Many CATV cables use this technique. This method works well as long as the cables are only flexed during installation and maintenance.
A served shield offers higher shield effectiveness than the film shield. It can be used when the cable is subjected to moderate flexing.
When the cable is subjected to flexing, a braid shield becomes the best choice. By choosing the right size of wire size, the braid offers the best shield method.
Caluculating the size of shield material
Calculating the size of the shield material used to be a chore. The formulas for calculating the braid construction form an Eigen value problem for which there is no finite answer to the calculation. Early engineers created tables of shield constructions and used them as a guide to designing the braid. Today computers can quickly make the calculation. The formulas can be found in cable design handbooks and military specifications.
The materials used for shields
Materials for shields include copper, tin plated copper, silver plated copper, nickel plated copper, high and low permeable irons, carbon fibers, tinsel wire and aluminum.
The choice of materials for the shield and the choices for insulation and conductor materials depend upon the environmental conditions to which the cable will be subjected.
Calculating the shield coverage and braid angle
The shield shall consist of a woven braid using strand material specified in the cable specification. Coverage should not be less than 85% for most cables, but may be increased to 90%. The angle of the braid with the axis of the cable shall lie between 20º and 40º for diameters up to .600 inch (15.2 mm). For diameters larger than .600 inch (15.2 mm), the braid angle may be greater than 40º. Percent coverage, K, and angle of braid, a, shall be calculated as follows:
K = (2F - F2) x 100
F = NPd/Sin a
a = Tan -1 (2π(D +2d) P/C)
Where:
F = Fill or space factor
K = Percent coverage
N = Number of wires per carrier
P = Picks per inch of cable length
d = Diameter of individual braid wire in inches
a = Angle of braid with axis of cable
D = Diameter of cable under the shield in inches
C = Number of carriers
Shield effectiveness is expressed in decibels or DB. For a single copper shield, the value is around 40 DB for 85% coverage and only climbs to 45 DB for 90% coverage. By using two copper shields, the value rises to around 60 DB. To go higher in effectiveness requires the use of high and low permeable irons.
For more information on shields refer to: MIL-DTL-27500
For assistance with custom wire & cable design, contact a design expert at
Calmont Wire & Cable, Inc.
Website: www.calmont.com
Email: Contact Us
Phone: 800-905-7161 ext. 135
Posted by Barbara Monteleone on Sat, Feb 19, 2011 @ 12:32 AM
CONDUCTOR SIZE FOR CONTROL AND INSTRUMENTATION CABLES
For a control or instrument device to function properly, it’s important that cables contain the appropriate conductor size. The standard ampacity tables found in electrical handbooks were created for 60 cycle power cables. (Ampacity is a measure of the current required to raise the conductor to the rated temperature of the insulation.) The standard ampacity tables are fine for power circuits, but they’re not valid for most control and instrumentation cables.
To calculate the conductor size you will need to know the maximum voltage drop that is acceptable for your application.
Example:
You must know the following:
•Acceptable voltage drop
•Current the wire will carry
•Length of the wire
Example
84 Volt Battery
Using Ohm’s law where: V=IR
V=Voltage (Volts)
I =Current (Amps)
R=Resistance (Ohms)
The maximum allowable voltage drop Vdrop = Vsource - V minimum load
= 84V - 80V = 4V
The current required is 3.7A
The total length of wire in this case is the distance from the source to the load and back again. In this case it’s 25 ft twice giving a total of 50 ft.
Given a current of 3.7A the maximum resistance allowable:
V / I = R 4 / 3.7 = 1.08 Ohms
Find a conductor that will have a resistance equal to or lower than 1.08 Ohms per 50 ft.
Wire tables always give the conductor resistance in Ohms per 1000 feet. We need to convert the 1.08 Ohms per 50 ft to the value of Ohms per 1000 feet.
1.08 Ohms/50 ft where 1.08/50 = .0216 Ohms per ft
.0216 Ohms/ft x 1,000 ft = 21.6 Ohms / 1,000 ft
From the Wire Table, select a conductor with a resistance lower than 21.6 Ohms/ 1,000 FT.
|
WIRE TABLE
|
|
AWG
|
Stranding
|
DC Resistance
(Ohms/M Ft@ 20̊C)
|
|
20
|
7/28
|
9.76
|
|
22
|
19/34
|
14.8
|
|
24
|
19/36
|
23.6
|
Note: M FT = 1,000 FT
Looking at the conductor tables we find that 22 AWG or larger will suffice.
The stranding will be determined by the application.
For some, it’s easier to understand the concepts of voltage, current, and resistance in term of hydraulics.
Voltage, Current and Resistance can be thought of in terms of fluids where:
Voltage (V) = Pressure
Current (I) = Flow Rate
Resistance (R) = Constriction
Another example:
We have a device to be attached at the end of a cable. It requires at 4.6 to 5.2 VDC supply at 120 mA. The source supplies 5.0V exactly. We need 65 ft of cable between the load and the source. What gage of wire should we be using?
Answer
1. Acceptable voltage drop across the cable is 5.0V - 4.6V = 0.4V
2. Current 0.120A
3. Wire length is 65 feet times 2 (for both ways)
4. R=V/I=0.4/.120= 3.3 Ohms
5. Total Resistance to device and back is 3.3 Ohms per 2 x 65', 3.3 Ohms/130 ft
6. 3.3 Ohms/130ft= .02564 Ohms/ft This is 25.64 Ohms/1,000'
7. From Wire Table find the conductor with a resistance of 25.64 or less.
8. Use 24AWG or Larger.
For assistance with custom wire & cable design, contact a design expert at
Calmont Wire & Cable, Inc.
420 East Alton Ave.
Santa Ana, CA 92707
Website: www.calmont.com
Email: calsales@calmont.com
Phone: 800-905-7161 ext. 135