What are Trapezoidal Screws?
Trapezoidal lead screws and nuts are power transmission screw threads similar to the Acme series but made to metric dimensions and standards. One such standard is the German Metrisches ISO-Trapezgewinde DIN 103 which has been adopted by the International Organization for Standards (ISO). Sometimes misnamed “metric Acme” or “metric M series Acme” Trapezoidal lead screws look like Acme screws and have the same stout thread form and the same robust and durable features as the Acme series.
Trapezoidal single start thread designations are the letters “TR” followed by the Major Diameter in mm and then the pitch in mm separated by an “X”. Thus, a “TR 14 x 3” Trapezoidal thread has a major diameter of 14 mm and a pitch of 3 mm. The separator “X” is commonly enunciated as “by” so one would communicate this size verbally as “Trapezoidal fourteen by three”.
One subtle but important difference between ACME and Trapezoidal thread is the flank angle. Acme threads have 14 1/2 degree flank angles (29 degrees included) whereas Trapezoidal threads have 15 degree (30 degrees included) flank angles. This means that single point tooling used to produce Acme threads both internal and external cannot be used in making Trapezoidal threads and vice versa. Users must be careful when measuring all thread features to determine whether a given specimen is Trapezoidal or ACME as the two can be easily confused. For example, 1 – 5 ACME has a Major Diameter of 25.4 mm and a pitch of 5.08 mm. It is quite easy to mistake this size for a TR 25 x 5 Trapezoidal. A visual examination would not be accurate enough to distinguish these two sizes – caliper, micrometer or thread comparator measurements would have to be made.
Advantages and Features
Roton’s rolled thread Trapezoidal screws and high performance Trapezoidal nuts are combined to provide the optimum performance available in this type of power screw. The highly burnished surface finish of the screw and low friction bronze and plastic nuts yield a smoother, less abrasive action than conventional cut Trapezoidal screws. Improved wear and surface properties insure users a longer service life and quieter operation, requiring less power in a wide variety of applications.
Materials and Processes
All Trapezoidal lead screws, 44 mm in diameter and larger, are made of special quality medium carbon steel. Screws smaller than 44 mm are made from special quality low carbon steel. Alternate screw materials are 304 type stainless steel, alloy steels, high machinability grades of carbon steel and aluminum alloys.
Trapezoidal lead screws are cold formed using a unique, highly accurate thread rolling process which guarantees a smooth, high quality thread. Roton’s thread rolling process eliminates the interrupted grain flow of conventional cut threads, thereby increasing yield, ultimate, and fatigue strengths.
With over 20 sizes in right and left hand versions, Roton offers the most complete Trapezoidal lead screw products made in the USA. The complications of freight, duty and long lead-times of importing these from foreign sources are eliminated. Standard nuts are offered in bronze and plastic. Bronze and plastic nuts are offered in universal sleeve style and in threaded mount style for mating with steel flanges.
All screws are furnished with a plain, oil dip finish. Screws may be electroplated, but use of stainless steel is encouraged for corrosive environments because electroplated surfaces are the first to wear off in use. Also, electroplating requires very close process control to avoid fit-up problems with mating nuts. Without extremely accurate control, the thickness of plating can vary greatly over the length of the screw shaft. Users intending to electroplate are encouraged to review Roton Engineering Bulletin No. 963 which discusses the plating process in detail. Black oxide finish is also available.
Most applications can be satisfied by selecting from the stock sizes listed. However, OEMs may find an optimum design solution requires a special size or special features such as stub series threads, special diameter and lead combinations, multiple start thread, etc. For these applications, Roton is uniquely qualified with over 28 years of experience in serving the OEM market. Extensive tooling and software resources can be utilized to assist the OEM designer in finding the very best metric solution for any application. Contact Roton Application Engineering for more details.
When selecting the right nut for an application, there are a few things to keep in mind. Bronze nuts provide superior service at high working loads. They should be well lubricated with a good quality, Extreme Pressure (EP) grease compatible with the operating environment. Plastic nuts can be used without lubrication and are popular for low load applications.
Application engineering information is available from Tables 21 and 21A. Additional engineering information is available in the Application Engineering Section. Those designers not familiar with power screw application are encouraged to review this section as well.
For those customers who wish to manufacture their own special nuts, Roton offers metric Trapezoidal taps designed and made for each specific application. Short leadtimes are possible using CAD and CAM capabilities. Our associated tap factories can usually design and deliver taps quicker than foreign sources without the importation delays and complications. For special Trapezoidal taps, contact Roton Engineering.
All Trapezoidal screws have lead angles of 5° or less making them self-locking. Trapezoidal screws will not backdrive under most conditions. This eliminates the need for brakes or other holding devices to sustain loads after they have been positioned. All other screws except Acme series require some braking means to prevent backdriving.
All sizes of Trapezoidal screws are made in large volume and their widespread popularity makes them the least costly of all power screws.
Mechanical efficiencies for Trapezoidal lead screws range from 20% – 40% depending upon the lead angle, coefficient of friction and nut material. Plastic nuts offer approximately 10% – 20% better efficiency than bronze because of their lower coefficient of friction when running against steel screws.
Low lead angles offer a high mechanical advantage for Trapezoidal screws. This means they require less input torque for a given load than other power screws. The finer the threads for any given diameter, the greater the mechanical advantage.
Roton’s Trapezoidal lead screws and nuts are widely used and well suited for both manual and power drive applications including:
- Pharmaceutical machinery
- Packaging machinery
- Machinery for global markets
- Slide drives
- Metric valve stems
- Medical device drives
- Farm machinery
- Screw press drives