Design Considerations

Cost
Reliability
Materials
Efficiency
Lead Accuracy
Thread Clearance
Lubrication
System Repeatability
Loading
Motor Sizing

TranTek realizes the decision to purchase a new type of drive system is an uncertain venture and would encourage its potential customers to consider every aspect. TranTek has developed a market for its drive systems by competing with ball screws, rack and pinion systems, and belt-driven systems. The DriTran epoxy composite nut drive system will provide superior service in applications where:

• Very long transfer lengths are necessary
• Extremely heavy loads must be transported
• Exceedingly high velocities must be achieved
• Screw wear and catastrophic failure is a concern
• Critical speed of the drive screw is a concern
• Lube failures are possible or likely
• Repeated machine crashing is likely
• Noise and vibrations are undesirable
• Machine tool tolerances are not necessary

TranTek drive systems perform reliably for extended periods under all of these conditions, where competitive systems are prone to failure and a limited service life.

Cost Back to top>
A DriTran epoxy composite nut and high lead drive screw system will increase the value of your design by speeding up processes, while simultaneously improving reliability. Cost savings will be realized because these drive systems will outlast other drive systems. Additionally, when a system does begin to experience marginal characteristics, the nut is the only part in need of replacement. TranTek suggests initially purchasing a spare nut to avoid unnecessary downtime, since nut replacement can usually be done in about one hour. Further cost benefits can be realized over the life of a DriTran drive system due to a unique feature which allows TranTek to rebuild a nut housing by re-casting the threads to like new specifications, normally, for less than the cost of a new nut.

Reliability Back to top>
The epoxy composite nut drive system is not a new technology. In fact, the casting method, which has existed since 1986, has been repeatedly tested refined and retested. Using a trade secret formula, it is a simple and repeatable process. Life and recast data is kept on every epoxy composite nut produced. With over 3000 custom drive systems built, TranTek drives have proven to be reliable in a wide variety of manufacturing applications and environments.

Materials Back to top>
TranTek high lead drive screws are very accurately cold rolled from low carbon steel, yielding high strength and superb wear characteristics. Rolled threads eliminate the interrupted grain flow found in milled, hobbed, or ground threads, resulting in increased yield, ultimate, and fatigue strength properties. Hardening the screw is not required because wear is confined to the nut. DriTran epoxy composite is a trade secret formula developed by TranTek and manufactured using very consistent and repeatable processes, insuring high strength and a predictable life.

Efficiency Back to top>
The highly burnished surface finish of the screw combined with the low friction properties of the epoxy composite gives TranTek drives very low start and run torques. With efficiencies of 70 to 80 percent when lubricated, these drives are not self-locking. Motors may need brakes to avoid back driving or to hold a load in place.

Lead Accuracy Back to top>
Lead error varies for each standard drive screw. Measured per linear foot, this lead error is cumulative over the length of the screw. The lead accuracy for the HSD143 series is ± .007" maximum; the HSD187 series has a ± .008" maximum, and the HSD276 series has a ± .009" maximum.

Thread Clearance Back to top>
DriTran epoxy composite is designed to wear slowly and to prevent damage from occurring to the screw. During casting, the master mandrel gives the nut .002" to .005" of linear thread clearance, providing sufficient room for lubrication. This thread clearance, or axial lash, increases with the life of the drive system as the epoxy composite slowly wears. Once the DriTran nut reaches it's maximum suggested thread clearance of .030", it can be replaced quickly, and the worn nut can be returned to TranTek for recasting.

Lubrication Back to top>
All standard TranTek drive systems come with an automatic lubrication device for the nuts and standoffs. In most applications, these drives require a steady slow supply of lithium grease to achieve their maximum life expectancy. Harsh environment applications also benefit from air knives and splashguards by reducing the amount of foreign particles, such as microscopic metal chips, entering the thread path. A unique benefit of a DriTran epoxy composite nut is its ability to survive extended operation during a lubrication system failure. As with any drive system, efficiency will decrease without lubrication, but a TranTek drive will not suffer a sudden catastrophic failure.

System Repeatability Back to top>
To accurately position a carriage, hard stops in conjunction with shocks can be used at the end of travel. Shock absorbing qualities of the DriTran epoxy composite and the large amount of load surface area mating the nut to the drive screw allow TranTek drive systems to survive hard stops in applications where a ball screw would rapidly fail. Accurate mid-screw positioning can be achieved, also, by mechanical means or with electronic controls.

Loading Back to top>
Drive systems always perform better when all loads are axial. Any off center moment, or radial loading will cause undue stresses, reducing the life of the nut. DriTran drive systems require external guiding of the nut to insure that all loads are axial. Also, as with any screw type drive system, tension loading is preferred. If possible, designers should locate the drive end of the screw such that compression loading is minimized.

Motor Sizing Back to top>
Designers must consider system inertia, acceleration/deceleration torque, and RPM requirements when sizing motors. TranTek High Speed Drives have about 80% efficiency, therefore, large loads will cause back driving of an undersized motor affecting positional accuracy. With many years of drive system experience, TranTek engineers can provide the necessary technical information to assist in motor sizing.