The "Spetsmashtrest" plan assigned the production of 510 vehicles to HPZ in the first half of 1936.
Only 425 tanks were produced in this time. ABTU accepted only 271 of these tanks. More specifically: out of the 100 tanks prepared in January, ABTU accepted 60, from 115 in February, ABTU accepted 99, out of 95 from March, 81, out of 51 in April, 25, out of 48 in May, none, out of 16 in June, 6.
The main problem is the low quality of a number of components in BT-7 tanks, reducing their combat quality. This is mainly caused by the design of the BT-7 using several components from the BT-5, which the factory transferred without consideration for the increased engine power and vehicle weight.
Data in our possession indicates that the chief of the tank design bureau at HPZ, engineer Firsov, removed himself from regulating work on this tank. All of Firsov's work was transferred to his assistant, Granberg, who had no experience in tank building, and did not make correct technical decisions in the design and construction of the BT-7.
The following defects of BT-7 tanks have been established:
- Incorrect assembly of the gearbox, resulting in mass breakage of the gears.
- The dust filter is inadequate. After 30-40 hours, the engine ceases working, due to dust in the cylinders and cylinder rings.
- Installing and removing the batteries is difficult. Instead of the required 8 minutes, it takes up to 2 hours.
- The removal of water from the radiators is inconvenient, plus it is impossible to purge all oil from the oil system.
- There are many cases of shorts in the electrical systems, increasing the chance of fire in the tank.
- Tracks are useful for 700-800 kilometers. According to the warranty negotiated with the RKKA, they should be functional for 2000 kilometers.
- Additionally, after 100 km, the track is loose and hangs on the wheels, slides off, and does not allow the tank to move normally.
- After ballistics tests of a 1935 hull and turret, it was deemed that the rivets are not bulletproof. A part of them was knocked out, the gas tank was penetrated, as were the hinges on the driver's hatch.
- Only one defect from other factories is worthy of attention: the use of ball bearings #6217 and #6411 from the 2nd State Ball Bearing Factory in gearboxes and gear change brackets. These ball bearings, due to metal quality and poor thermal conditioning, cannot be used on tanks. All ball bearings on tanks at HPZ were replaced, but the tanks in the army still contain 2nd GPZ ball bearings.
- Rapid wearing of the engine components due to a poor dust filter.
- The rollers on the gearbox bend and vibrate during operation.
- The final drives leak. It is necessary to redesign the final drives.
As for the issues communicated by comrades Gai and Mazo, we consider it necessary to report the following:
Gearbox: in April of 1936, it was determined that the conical gears of the gearbox are destroyed before the warranty period expires when operating on tracks. When this was discovered, HPZ immediately stopped sending tanks, and tested 13 tanks on tracks. 7 tanks did not manage the warranty period, only making it 500-700 km on tracks. These trials demonstrated that the cause of these breakdowns is the increased momentum of the M-17 engine when compared to the Liberty engine.
HPZ used all possible resources to make 9 design changes to the gearbox, increasing its strength 2-2.5 times compared to the May production gearbox.
At the same time, in order to increase the warranty period, according to the professor of the Academy of Motorization comrade Stepanov, the factory decreased the momentum of the M-17 engine from 280 kg-m to 220 kg-m.
Four BT-7 tanks with reinforced gearboxes and 220 kg-m moments were tested at great length by the factory and ABTU. Trials were performed in most difficult conditions for the gearbox, exclusively on tracks (the wheel load on the gearbox is 2-3 times lighter). Even in these conditions, the two first tanks travelled 1400 km, and the second two, 2000-2500 km. These trials convinced ABTU of the combat ability of the fast convertible drive BT-7 tank with the reinforced gearbox, which is capable of providing 2000 km of service, of those 1300 on tracks, which is acceptable by standards set by ABTU for 1936.
As for the history of the BT-7's development, we report that as of January 15th, 1935, the factory and ABTU completed an experimental BT-7 tank, that had an identical gearbox to the BT-5. That tank travelled 1100 km on tracks and 1300 km on wheels. A second identical prototype was tested by ABTU on an obstacle course.
- The conical pair component must be altered according to blueprint #08-S23.
- Produce an experimental prototype of a further reinforced version of this component, according to diagram #P-11-1"
- An increased number of retention bolts, from 10 to 12, with an increased diameter of their location (110 mm to 135 mm).
- The roller clamps are reinforced, 17 mm from 13 mm.
- The gear clamps are reinforced, from 13 mm to 11 mm.