| When it comes to specifying a Roll-Ring®, Diamond has a library of designs that are available in their existing configuration or that may be modified to meet your specific requirements. Diamond engineers can also create new designs to meet the most demanding applications.
Diamond’s engineers bring a wealth of design experience and the engineering knowledge required to tackle any application. Diamond partners with its Roll-Ring customers to understand their application, constraints and performance requirements in order to meet their technical and programmatic challenges.
This article is designed to guide you through the process of specifying a Roll-Ring. The major design and program considerations that our engineers need to know are outlined below.
Roll-Ring Basics
Roll-Rings are based on the rolling contact of flexures between grooved rotor (inner) and stator (outer) rings. In a manner similar to a planetary gear, the flexures (shaped like a wedding band) orbit the inner ring while maintaining a very low wear, rolling contact between the inner and outer ring.
There are two basic styles of Roll-Ring products: Roll-Rings and Roll-Blocks.
1. The original Roll-Ring consists of “stacks” of stator and rotor rings, where each ring is adjacent to the next along the centerline axis of rotation.
2. Roll-Blocks are the next generation Roll-Ring and consist of a Roll-Ring embedded into a conductive wheel. The conductive Roll-Block wheel rolls on the surface of the rotor, completing the circuit.Roll-Blocks may be used with either drum style rotors or platter style rotors with concentric conductive rings. In either configuration, in new designs or as retro-fits, Roll-Blocks are a high performance alternative to slip ring brush blocks.
Defining the Mechanical Envelope
Every application has different constraints and the first constraint that must be considered is the mechanical envelope. The mechanical envelope defines the space (volume) that the Roll-Ring must fit within. The mechanical envelope also defines any exclusion zones such as hollow bores along the centerline axis of rotation. Additionally the maximum allowable weight should be defined as part of the mechanical envelope.The mechanical envelope will be the first criteria used to evaluate the applicability of existing designs. Whenever possible the maximum mechanical envelope should be defined in order to maximize the candidate pool of existing designs.
Defining the System Interfaces
Whenever possible an interface control drawing should be provided in order to fully define the mechanical envelope and other interface requirements. Typical system interfaces include:
1. Ownership of the centerline axis of rotation on open bore designs. Is the open bore needed to mount additional components such as a rotary joint (RJ), fiber optic rotary joint (FORJ), or a fluid rotary joint (FRJ)?
2. Mounting requirements that define how the Roll-Ring will be connected to the rest of the system. Details such as flanges and bolt-hole patterns are usually defined here. It is important not to rigidly mount both the rotor and stator ends of any rotary joint since any misalignment will overload the bearings and result in damage.
3. Wiring and connector requirements that specify how the electrical connections are made. Many high performance applications have specific wiring requirements that need to be considered early in the design process. The connector requirements are especially important. Should integral connectors or flying leads be provided? What orientation and location is required (axial / radial)?
If flying leads are required, what are the cable lengths? What are the labeling requirements?
Defining Electrical Requirements -Power
and Low Data Rate Control Signals
Definition of the electrical performance requirements is critical. Load current requirements drive flexure diameter as well as the number of flexures per circuit that is required. The circuit voltage requirements drive the insulator thickness. Together these requirements will determine the axial length of the Roll-Ring.
A detailed understanding of the application and the duty cycle is required in order to avoid over-specifying these performance parameters. Proper specification of currents and voltages enables Diamond engineers to provide cost effective solutions in the minimum mechanical envelope.
The bottom line is that everyone pays for performance. Take for example:
- A 10 circuit Roll-Ring with:
- 3 - 20 A high power circuits
- 3 - 5 A low power circuits
- 4 - 0.5 A data or signal circuits
Specifying all 10 circuits at 20 A in order to have 100% interchangeability will result in a larger, heavier, more expensive design. Additionally if 20 A is a surge current value and 10 A is the continuous current specification; this would also result in a larger, more expensive design.
It is helpful to identify if there is a critical requirement for the circuit resistance or the resistance variation. For example, circuit resistance can often be tuned by selecting the wire or flexure size based on resistance instead of current handling capability.
If you have any questions about the proper specification of the current and voltage circuits contact a Diamond engineer. Diamond’s engineers will work with you to properly document your application and identify your requirements.
Defining High Data Rate Circuits
Modern control systems require the capability to conduct high bandwidth, high frequency analog & digital signals in addition to the traditional low bandwidth control signals. High frequency data transmission rates are driven by several factors:
- Insertion loss
- Impedance of the assembly
- Impedance of the cables
- Differential time delay
- Crosstalk between circuits
- Frequency response of the cables and connectors
Frequency response (or bandwidth) is the primary factor to consider. Errors reduce performance and are introduced into digital data streams when a digital signal is distorted in either amplitude or phase. This distortion results in data or timing errors within the digital system. Digital signals are composed of multiple harmonics as multiple analog signals are combined to define digital square wave pulses. Therefore the bandwidth requirement of the Roll-Ring is typically several times the data rate, e.g. a 10 MHz data rate may require a 50 - 70 MHz frequency capability in order to maintain optimum pulse shape.
Frequency defines wavelength which in turn defines the size of transmission components. Therefore high data rate products generally require smaller diameter Roll-Rings. Design of high frequency components is a complex task. Diamond understands these complexities well and leverages its 50 year history of designing high frequency microwave rotary joints in the design of high data rate Roll-Rings.
Given that there are some wavelength defined performance constraints that must be considered it is not sufficient to simply specify “10 Mbit Ethernet” for high data rate channels. It is critical that the entire system be understood and specified since it usually requires a compromise between performance, size, weight, number of channels, the external environment and cost.
The following parameters should be specified to enable Diamond engineers to evaluate different designs:
- Data bus used to transmit data, i.e. 10 Base T or 100Base T Ethernet, Firewire
- Cable and connector types used
- Maximum cable length between transmitter and receiver
- Maximum data rate
- Maximum error rate that can be tolerated
For the most advanced and challenging applications Diamond integrates fiber optic rotary joints (FORJs) into our Roll-Ring designs. Diamond works with its FORJ partner to provide single and multi channel FORJs. The FORJ is used on the very high data rate channels or on circuits requiring very low crosstalk. In these applications the Roll-Ring would be used to transmit power and more traditional, low data rate control signals. Diamond and its FORJ partner can also provide the hardware to perform the EO/OE (electrical to optical) conversion.
Defining the Mechanical Requirements
The very low wear resulting from rolling contact of the electrical interface provides Roll-Rings one of their greatest performance advantages over alternative technologies such as slip rings. Important mechanical considerations include:
1. Operating speed (rpm) is an important design parameter because it helps drive bearing selection which in turn drives costs.
2. Duty cycle of the application – This is an essential parameter to understand when selecting components such as bearings and seals.
Duty cycle questions to consider include:
- Will the unit rotate at a single continuous speed? Will it do so 24 hrs per day / 7 days per week / 365 days a year?
- Will the unit rotate thru several different speed & duration profiles? If so how often do they repeat? Is there a period of static operation? Is direction of rotation constant?
3. Design life or target MTBR / MTBF (Mean Time to Repair / Mean Time between Failures). It is necessary to understand the design life and how it is measured. Used in combination with duty cycle it allows Diamond engineers to make cost effective design trade offs.
Defining the Environmental Requirements
The environment in which a Roll-Ring must operate and the extremes it must survive are also key design parameters. This affects many features of the design from material selections to bearing lubricant.
Questions to consider and ranges to define include:
- Will the unit be operating in an extreme temperature regime?
- Will the unit be exposed to radiation?
- Will the unit be exposed to the elements (rain, snow, high humidity, dust, …)?
- Will the unit be exposed to shock or vibration? What type (road, drop, gun fire…)?
Defining RoHS / WEEE Requirements
Starting on July 1, 2006 the European Union’s Restriction on Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) go into effect. These regulations ban the use of seven environmentally sensitive substances. Some industries and/or applications are exempt, some are not. If your product is impacted, Diamond will work with you to provide the required certificate of compliance for Roll-Ring products. Roll-Rings contain no mercury.
Beyond the Sale - Program Requirements
Diamond partners with its Roll-Ring customers to understand their applications, constraints and performance requirements in order to provide high performance, high value-added components. But a good partnership goes beyond the technical specifications.
That is why Diamond engages with the entire organization. In order to meet ever increasing quality standards such as Six-Sigma many organizations have defined stringent acceptance testing and product qualification processes. Other organizations are not as formally structured or industry standards are not clear. These customers often rely on Diamond’s expertise and guidance in these areas.
Questions to consider include:
- Is there a formal process to approve a purchase specification? Is one or more design review required (for a custom design)?
- Is a first article report required on the base technology or each design?
- Is a vendor audit required? If so how often?
- Is there a defined Acceptance Test Plan (ATP)?
- Is there a defined, stage gated Product Qualification process? If so is there a clear predefined Qualification test requirement?
- If MIL specifications are referenced - is MIL test certification required?
- Are there regulatory hurdles (such as licenses, CE Mark or UL approval) that must be addressed?
Diamond’s engineers work with every department in order to clearly identify all of the programmatic requirements and provide the highest level of customer service possible.
For assistance specifying your Roll-Ring requirements please complete a Roll-Ring specification sheet.
|
